Generally speaking, get the biggest, with good quality, TV you can afford. You will always wish you'd gone bigger.
I currently sit 11.5' from a 106" 720P HD screen and will be stepping up to a 120" when I go to a 1080P projector soon.
When viewing a display you're about to purchase, bring a tape measure and view from the distance you'll be sitting at home if possible. 42" may seem big from 5' away at Best Buy, but will seem smaller at home on the opposite wall.