There are a couple of measurement options that can get really mathematically cumbersome, but I like the Rule of Three. For every inch on the panel, you should sit back 3. So a 55 inch would be pretty significant for a person sitting back 9 feet. Then again, there are lots of people who don't care, because they want a big TV and they like it that way. And as you get older, your eyesight might warrant a bigger screen. But this is really a personal preference, and the whole issue is a preference determination.
The reason I recommend against too large of a screen is that your eyes can't fully encompass the picture without moving, so your eyes will go back and forth and give the viewer eye fatigue. In my opinion, it takes away from the experience of watching the TV over long periods of time. I sit the same distance from a 46" and my eyes don't move very much, at least not as far as if I was sitting closer to it.
Does that help?
Samsung used to have a TV size decision maker on its website but I can no longer find it. Some clown is trying to sell my OLD man a 55 inch 3D TV for a room where he sits about 9 feet from the screen.
Anybody have an ideas?