Look, we have detailed threads on this forum and over at HDGURU.net showing the optimal distance for various sizes, 1080p vs. non-1080p, etc. In the final analysis, these things are a GUIDE and it also depends on your personal preferences.
Let me say one thing, and I say this as a guy who believes in 'bigger is better' and wants to up-size my plasma in the future: Alot of the impact on what size you will get not only is based on viewing distance charts but also on WHERE YOU ARE COMING FROM CURRENTLY with your existing setup.
If you are watching a 27" CRT from 8-10' and goto a 42" plasma as I did, you're probably going to be very pleased even if the charts say that at that distance (or a bit further) you really can take in a 50". And of course, those people who love the immersion factor -- like the people who sit in the front row at a movie theater -- will always go for the biggest, irrespective of distance.
The point is, take into account what your current setup is now. For years, I had nothing larger than a 19" TV so when I got my 1st 27" back in 1995, it seemed HUGE in my bedroom from 6-7'. The 27" in the living room from 8-10' was a bit 'smallish' but still not bad. But now that my eyes have adapted to viewing a 42" 16:9 display, if I were to put my old 27" back on the stand I'd be looking at all the surrounding stuff in the background, since my eyes are now used to viewing a certain screen sizse. Heck, when the 27" had to be repaired and I put in a 19" I kept 'overviewing' the entire TV and taking in the drapes, cabinet, etc. You'd have thought I had replaced the 42" with a 9" portable !
So, sure, use the viewing charts, and also take into account viewing distance and the fact that what may seem 'big' now might not be as big a few years down the road. But also take into account that if you are torn between a 42" and a 50" and money is a factor, and if you're coming from a 19" or even 27" CRT, unless the distance is alot greater you will be getting a big enough OOOMPH from even the 42" size.
JMHO, thanks for letting me rant.