I believe I read that a good ratio is 1.5x the screen size. So if you have 55 inch tv, you should sit back at least 82 inches or about 7 feet. I believe I read this in consumer reports.
I think I read that it's best viewed from 2.5 to 3 times the diagonal screen size. Another thing that I read said that you should sit just far away enough so that you dont see the resolution lines. Heres a link that will help with some of your setup and other questions... http://www.htadvice.com/on_setup/tvs_placement.html