Determining the correct screen size is just a matter of knowing the distance from your eyes to the screen.
An HD television should occupy no less than 24 degrees of your field of view. SMPTE recommends that it occupy 30 degrees of your field of view. If you want to literally see every detail possible (and you have 20/20 vision), you should opt for a 32 degree field of view. THX says to go with a 36 degree field of view. And a full-sized theater can be a 40 degree field of view, but that is typically based on 2.35:1 aspect ratio movies, which are much wider than HDTV's 16:9 aspect ratio.
So...figure out the distance (in inches) from where your eyes will be to where the TV screen will be. Remember to account for things like the thickness of the wall mount and the thickness of the TV itself.
Take that distance (in inches) and divide it by 2.05. That will give you the minimum screen size that you should consider. Any smaller and you are basically "wasting" having HD resolution. Divide the distance by 1.626 to get the SMPTE recommended size. Divide by 1.52 to get the 32 degree "maximum detail" field of view. Or go right up to THX size by dividing the distance by 1.34.
You will typically find that the screen size is larger than you might have thought. Of course, you can be limited by the available space on your wall or shelf or you can be limited just by aesthetic appeal. But if you really want to know what size TV you SHOULD get, figuring out the distance from eyes to screen and then getting the correct field of view is the way to do it
I have a 60" Kuro in my 12' x 14' theater and I sit a mere 7.6 feet away from it. That gives me a 32 degree field of view though, and since I want maximum detail, it's the right size for me