For a rear projection tv the ideal seating distance would be 5x the viewable screen height. For example if you own a 50in RPTV and the viewable screen height is 30in, than your ideal seating distance would be 5x30 or 150in or 12ft - 6in. After this advice read Sean's post.
How big is too big?
There has to be some sort of ratio of room size to screen size. I was thinking about getting an 84" screen but realized that it would be WAY TO BIG for the room after drawing it on the wall with chalk. Have any of you put too large of a TV in a too small room, or the other way around?