from THX
If your HDTV is going in a living room with limited space, THX recommends the following:
35 inch class TV = 3.5-5 feet away
40 inch class TV = 4-6 feet away
50 inch class TV = 5-7.5 feet away
60 inch class TV = 6-9 feet way
It might seem that THX is recommending a very large screen size for your room. But, it’s important to note two things: 1) THX screen size recommendations are based on recreating an immersive cinematic experience, and 2) the biggest complaint among TV buyers is that they wish they had bought a bigger set.
Vertical Placement
Make sure each seating position in your home theater has a clear site line to the screen. Try not to place the screen too high on the wall – viewers should not have to look up more than 15°.