My living room is smaller then yours and I just when back to a 50" plasma. I dont see why that would not work for you as well if your viewing distance is 12-13 feet away.
Just keep in mind that what your viewing also matters. HD and DVD materials have enough resolution which allows you to be closer if you like. However, regular television broadcast can be bad to down right terrible depending on either the channel or your service provider.
Considering a flat panel for my living room which is approx 17'x14', the best place to mount TV is in a corner & our normal viewing distance diagonally is approx 12'-13'. I thought of 32" flat panel (LCD) but I am thinking they might be too small, I don't know about 37" , 42" or higher. I've tried looking them in stores, but found tough to judge. Any suggestions please.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic