My seating is anywhere from 6 to 8 ft from my 46" plasma and that screen size works out very well for me. I have a very small den 10X20 Ft and any thing much larger would overwhelm the room. That's not saying larger wouldn't be better, but your over all room size among other things, should be taken into consideration. Also, the SMPTE and THX recommendations may be worth considering in a perfect world where you have perfect picture quality, but keep in mind, due to data compression and other HDTV anomalies, certain artifacts such as video noise and banding will be more noticeable the closer you get and the bigger the screen.
Ian