Yea sizes are a bit subjective. To me, small is less than 3k cu ft, medium is 3-5k cu ft, large is 5k+. However, I wouldn't put too much stock into that. There are a lot of different variables that go into how much sub you need for your room/personal preference.
Interesting. My room is 15.5x23.5x9.5, about 3500 ft^3. I always thought it was a fairly big room for a house. Size makes it easier to adjust, I think, but I guess I've never tried a really big room.
I'm using two ULS-15s & HB-1s in the front corners. The room has some interesting standing waves but I was able to get it down to a 5db range using Room EQ Wizard. It would be 4db but there is a peak just above 20hz I can't seem to control. I don't think I could do this in a much smaller room, but maybe I'm wrong.
Yea it depends on a lot of different things. In general the larger room you go the less likely you'll get room gain, but there are a lot of variables in that. Our room is about 4000 cu ft or so and we wound up getting a 7 dB boost in the ultra deep bass.