Depending on what kind of tv you are getting will determine what size you should buy. An CRT display will allow you to have a screen size that is 6 times smaller than the distance from where you sit. A HDTV will allow you to go as low as 1/5 - 3 times smaller than the distance you will be sitting from the TV. You can find more information here:
. http://www.life123.com/technology/home-electronics/hdtv/whats-the-right-tv-screen-size-for-my-room.shtml