How big is it?

I read a lot about "big rooms" and "small rooms" when it comes to speakers, and they say room size matters, but how do they measure? My listening room is 21' x 24' and in my neck of suburbia it's a very large room, but they guy on the hill might consider it a closet. So when we're reading about a speaker needing a large room or small room, just what do they mean?
Largely, room size has to do with the spkrs size. A room's dimensions are important (see cardas' site and immedia's) to minimise/ control standing waves.

Now, a BIG room is a relative matter -- no. The biggest I have seen/ listened in is ~2250'sq. Cheers
A 'tiny' room is a 'small bedroom' 8' by 10' tiny speakers.
A 'small' room is a 'small apartment' size 10' by 12' to 11'.
An 'average room' is 12' by 16'
a 'larger' room is 12' by 20' or 14' by 18'
a 'big' room would be 14' by 26' etc
a 'really big' room is 16' by 30'
a 'giant room' is 20' by 40'
anything bigger is the inside of a small concert hall!
Feel free to apply my definitions to your room.
My 'small' apartment main room is 11' by 32' (because the kitchen/dining area, which has only a low 1/2 wall partial divider, lies straight back of the long axis of the actual living room space... thus giving me a much larger acoustical space to listen in)
My Maggies are 6' from the back wall, and I sit about 8' to 10 ' from the speakers.
I would agree with Elizabeth and say your room is on the large size, so buy accordingly.
Smaller room has bigger possibilities for bigger sound.
Headphones have the biggest possible sound you can get.
Let me add to the question. What if half of the back wall in the room is open to the kitchen or another room? Are they considered seperate rooms or do you combine the footage?
Personaly I would not sum the rooms. I would worry more about your speaker and seating position in the primary room than buying speakers that will produce a given spl in the adjoining room unless that is an important factor. I know others probably have more knowledge than I regarding acoustics in rooms, hopefully they'll chime in.
I had to calculate the reflections from the "main" back wall (the kitchen wall at 32 feet) and the partial wall dividing the kitchen/dining from the living room.
So when I used one of the popular methods, the 'short' wall was 'bad'... then I found another method, and got some good results.. both the short wall, and the back wall are at resonance minimizing distances from the speakers.
(and don't ask what method.. I forgot)
I would say, the back wall IS the more important wall in your calculations. As the bass waves are what you really want to control the most, and the full wall is the thing that matters. (unless the short wall is at a killer standing wave for that speaker location)