The other key issue is how big a wall you need for everything that has to be up front. In addition to a screen, you have to accomodate speakers, including possibly a sub. To have a 55" screen with adequate placement of speakers, etc. you probably need on the order of a 12' uninterrupted wall, which isn't always easy to accomodate.
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?