In terms of size, try using the math formula someone told me about.
Basically, with a HDTV you simply measure the screen (n inches) and then work out if you can comfortably sit between 1.3 - 3 times that measurement away from it.
If you can then go ahead. If not, then sorry but your room is too small
I agree that the sound should match the size - too much in a small room seems pointless.