Try this calculator ... it is what I use ...
So, the calculator reveals the following for a 61" (assuming you are watching 8HDTV) ...
6.8 Feet Recommended THX viewing distance (36 degree viewing angle)
Viewing Distances Based on Visual Acuity
8 Feet Maximum Viewing Distance for HDTV(Fully resolved 1080i; 1920 x 1080)
I am about to purchase a 60" 1080p plasma ... 8 feet is distance I am going to use.