Size/Height of Rooms

There are two concerns: practical and aesthetic.  The practical is that the room has to be big enough to hold the furniture and still have space to move.  The aesthetic has to do with how the room feels, which is a function of the relation of its size to our body height.  On the practical side, rooms will often either have to grow or shrink from their desired size in order to make them all fit within the desired envelope, although sometimes it makes sense to bump out an exterior wall a little bit to allow a room to be a little bigger than it otherwise would.

Practical

Use a typical size

The simplest way is to use typical sizes, keeping in mind that it generally makes more sense to put space into common rooms (kitchen living) that private space (bedrooms, offices).  Typical room sizes are as follows (ie the majority of rooms will be in this range, but a significant number will be either larger or smaller).

Bedroom/Office	11 x 10 to 14 x 13
Kitchen/living/Dining	10 x14 to 15 x 16
Bath	5 x 7 to 8 x 12
Great Room	13 x 30 to 16 x 40

Note that one dimension is almost always never greater than 16' because this is the greatest length that common building materials can span under most typical conditions.  This constraint applies to not just the floor, but to whatever is above.  Needless to say, there are many caveats to this, and there are also work arounds, but they tend to increase the cost.

Ideal size

Rather than use typical size, it also possible to start out with a "desired" size, which is enough room to fit the likely furniture (typical sizes below), room to pass around it, and room for doors and windows.  Desks and tables need additional space to accommodate someone sitting and still have room to move past.

 The size of furniture tends to be fairly standard, with some variation, particularly for beds and dining tables. Walkway space can vary from 18" wide (barely passable) to 30" wide (comfortable passage) to 36" wide (big enough to walk plus move things thru) to 48" wide (very generous).   It is useful at this point to consider how the room might be used in the future--while bedroom designed for a single bed will not likely ever fit a king size, a room designed for a queen could be made slightly larger to accommodate a king.

The following chart gives typical approximate furniture sizes, however keep in mind that its not hard to find furniture that is outside of these typical ranges, so if you have a specific piece in mind, you probably want to measure it.

Approximate Furniture Sizes

Stuffed Chair	3 x 3
Love Seat	3 x 5.5
Couch	3 x 7
Breakfast table	3 x 3
Small Dining Table	3 x 5
Dining Table	3.5 x 6
Extended Dining Table	3.5 x 8
Single Bed	3.25 x 6.25
Double Bed	4.5 x 6.25
Queen Bed	5 x 6.7
King Bed	6.3 x 6.7
Small dresser	1.5 x 3
Large dresser	2 x 5
Small desk	2.5 x 4.5
Medium desk	3 x 5
Large desk	3.5 x 6

The best way to size rooms is to draw them to scale--use cut out pieces of paper to represent furniture--the catch is that you'll have to know what furniture you want and have some idea of how you want it arranged.   The example at right is an 11x12 bedroom with a queen bed and two small night tables.  The entry is at the lower right and a possible closet door at the lower left. There is still room to put a dresser on the wall at the foot of the bed, and currently there is more than ample circulation space.  If the location of the closet moves, the door might have to be bifold, or sliding, or no door at all, otherwise when open you won't be able to get past it.

 

Next, consider alternative arrangements of furniture that you, or a future owner might want and if the room needs to be slightly bigger to allow for that, make is slightly bigger.  In the case of the example above, the room can fit a king bed, so its adaptable as is.

Eventually the rooms all have to join to make a whole--room that don't have critical sizes might shrink a bit, while others will likely grow.  As a rule of thumb, its always best to start as small as possible, because everything tends to grow as it comes together in a whole.

Each room can be sized to its smallest reasonable size, independently of all other room, and then potentially resized as it is joined up with the other rooms to make a house.  In this process, rooms whose size is critical will stay the same size, and the others will grow or shrink accordingly, or alternatively the extra space will end up as storage, wider halls etc.  Obviously don't start with the room smaller than you're comfortable with, but its wise to start on the small side, since as the rooms are joined, every tends to grow, simply because its the easiest way to make everything fit.  It is often possible to keep things from growing much, and doing so will likely make a better building, but doing so will require much more design time.

Aesthetic

Just because a room is big enough to function, doesn't mean you'll want it that size.  People tend to judge rooms as cozy, claustrophobic or cavernous  based on both their own bias and the relation of the room size (including ceiling height) to their body height.   Whether a room feels cozy or claustrophobic also depends on its function--in a bedroom small might be desirable, but in a living room it might not be. because in the bedroom occupants expect a higher level of intimacy than in the living room.  Light and view can change the size a room feels--a view to outside, or opening allowing a view into an adjacent space (like opening a kitchen to the dining area) can make the space feel bigger than it is.

The perceived size is straightforward: most people will find a 10x11 living room to be quite small, while something more like 13x15 would be ample, 15x23 would be large, and 18x30 would be enormous.  However, room dimensions alone don't tell the whole story.  If we start with an 11x13 living room (still on the small side),  then join it up with  a dining area and kitchen into a 13x30 great room it won't feel as small because the room is now able to visually expand into the other spaces even though the furniture is confined to the smaller space.  A large window¹ or French doors can also serve to make a space feel larger, by opening up views to the outside.  As long as the ceiling stays lower than the shortest room dimension, a taller ceiling will also make a room feel larger-just moving from 8 feet to 9 feet can make a big difference.

Dividing a room up, via a wainscot, a partial room divider, or even just a lot of cluttered furniture will tend to make the room seem smaller, although surprisingly a completely empty room (unless its quite large) tends to look small because there is nothing there to judge its size by.

The sense of intimacy obviously is affected by the number of people in the room, how close you are to them, and how large a bubble of personal space you want.  What is less obvious is that people are often aware of even the potential of someone else being there, so that if they want to be alone, they will desire a space that avoids this potential for contact and if they want company they will gravitate toward such spaces.  In commercial buildings, especially ones where strangers are expected to sit near each other, the ceiling height is often quite high to make those spaces feel larger, and hence the physical separation between people also feel larger.  Since occupants of a house aren't generally strangers, rooms tend to feel smaller.

Noise has a very big footprint, and no reasonable size room in a house is big enough that any noise won't dominate the room.  However, when people engage in noisy activities like playing a game or watching TV they tend to require less personal space than when doing a quiet activity like reading: most people are willing to sit quite close when watching TV, but will want just a bit more space when reading or having quiet conversation.  Because of this, a Den which contains a TV can be smaller than living room where everyone sits around reading the Sunday paper. 

Another way to look at size, and likely we all do this subconsciously, is to view all sizes as related to our bodies size.  In this view, a small room would be anything up to 2x2 body heights, a ample one would be push closer to 2.5x2.5 and so on.  Obviously a larger person will see a room as being smaller than a small person would.

Generally, the common areas should feel larger than the more private space as this is where guests will be, but of course there is a degree of personal choice in this also.  Bedrooms, on the other hand are best left smaller so they feel more intimate.  One can take this a bit further and lower the ceiling over the bed even lower yet for a further sense of intimacy.  Its important to get in the ballpark of the right size: if you feel like someone else is always in your space, you'll feel cramped, even if the person is your spouse.  On the contrary if the room is so large that no one every seems to come near, then the room will feel cold and impersonal.

There are numerous theories about the proportions of a room as well, most of which boil down to "rectangles are good, but don't make them too skinny".  In most cases, the length should never be more than 1.5 times the width, and more typically it would be 1.1 to 1.3 times.  This isn't to say that square is necessarily bad, only that square rooms have a tendency to feel a bit more awkward--however this is quite minor, and in practice most rooms don't end up square anyhow.  Unless a room is grossly out of proportion, this issue of proportion can probably be safely ignored.

The only other main consideration of room size is daylight.  Since daylight will only penetrate ballpark 2 times the distance from the floor to the top of the window, room size is generally limited to being no more than 14-16 feet wide from windows to back wall.  Conveniently this is also approximately the maximum span for common floor joists.

Ceiling Height
The height of the ceiling affects both the size a room looks and the size it feels: a taller ceiling will tend to make the room look bigger than short one.  The caveat is that the ceiling height must always be less than the room's shortest dimension, otherwise a taller ceiling will actually make the room look smaller by making it obvious that a dimension is short.  While its obvious that a taller ceiling actually does make the room bigger (in volume), it turns out that it also makes it feel less intimate: if you're sitting six feet from someone, you feel at lot further away in a 50x50 room with a 30' ceiling than you do at the same distance in a 15x20 room with an 8' ceiling.

 A one foot difference in ceiling height impacts the size a room feels much more than an extra foot of floor space.  Four feet of extra height is likely more dramatic than making one dimension on the floor four feet longer. The eight foot ceiling was adopted for residences because its a good compromise.  The common areas could benefit from a slightly higher ceiling (maybe 8'6" or 9, but probably not more than 10) while bedrooms, bathrooms and home offices could easily have a slightly lower ceiling (by probably not less than 7'6").  When a taller ceiling is used in the common areas, many people will keep the kitchen ceiling lower.

Vaulted ceilings under a roof can be nice, but you need to keep in mind that warm air will rise, so if the ceiling gets too tall, the warm air will all go up there.  There are few practical reasons for a ceiling much more than ten feet, although a sloped roof starting out at a height you can touch (or nearly so), could average a bit more than 10' high and feel OK...although at some point around there the sense of being covered by a roof goes away.  The one case for a tall vaulted ceiling would be to specifically capture hot air² either to move to a higher floor or to send it away up out the roof.. Ceilings two stories tall not only cause a problem with heat, but they're a colossal waste of space.  Cathedral ceilings are best left for churches and train stations, where they belong.

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Notes

1: But of course you have to consider the energy advantage/penalty of putting in a large window as well.

2: this isn't to say there aren't some other reasons, just that I've yet to find them.  From the green building perspective, cathedral ceilings are a huge waste of space: the ceiling height doesn't scale with the room, and there is a lot of materials going into building space that holds only unused air.