Engadget HD has a great post about finding the right size HDTV for your room. It breaks down at what distance the benefits of a certain resolution become apparent.

In my particular situation, Id have to get a 56'' TV before the benefits of 1080p became really noticeable from my average viewing distance.

If you're thinking about buying a TV in the near future you should definitely save this guide as a reference, no need to waste a ton of money on a 1080p set if you won't be able to notice a difference from 720p

Check it out - Engadget HD