Size/Height of Rooms

For the most part, the size of a room should be determined largely by the furniture that goes in it combined with the necessary floor space surrounding the furniture.  Start by determining the minimum size by placing the furniture in the room where you want it, then adding the necessary space around it. Most furniture is fairly standard size, and pathways around 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).  For furniture whose size varies quite a bit like beds, decide on what the largest size you want to accommodate, typically either 5' for a Queen or 6'6" for a King.  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.

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.

The remainder of this discussion is about how size and proportion affects the feel of a room.

The Size of Rooms
Once you've determined the minimum practical size for a room, the next thing to consider is how big the room feels.  Since this is about human perception, there is no fixed answer as everyone will see things a little different; however there are still common threads.  There are two obvious aspects of perceived size: first is just what the eye sees, that is the size a room appears to be; second is the size the room feels, which is a sense of intimacy-- that is whether or not you feel close to others, which in some situations is good and in others you feel invaded.

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.

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.


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.