My husband wants a 50" plasma, but I think anything bigger than a 42" would look ridiculous in our room. It's an older house, so the tv would only be 10 feet or so from the couch. Isn't there a rule of thumb on this?
I sit about 12 ft from a 60" set and wish it was bigger..lol. One thing to consider is that widescreen tv looks a lot smaller with the side bars when watching regular tv. At 10ft a 50" is fine in that room. Here is a link to cnet for viewing distances. Enjoy that 50" tv, cnet says its just right for that room :).
Here is a link that might be useful: Size up your screen
For watching movies on DVD or watching true HD TV programming, I suggest a screen width that's half the distance to prime seats, or perhaps a little wider. That's assuming your plasma will be HD capable (are there any models that aren't??)
The rule of thumb used by architects designing high-end home theatres is that the screen should fill about the same proportion of your field of vision as would a movie screen when you're sitting ~ 2/3 back from the front of the theatre. It will give you the "big screen feeling" of a real movie theatre experience, but you will be far enough back from the screen that normal image defects (and all TVs/projectors, etc. have them) won't be noticeable and spoil your experience.
All of this assumes that you'll also have the correct DVD player, wiring, cable/satellite box, cable/satellite signal and feed, etc. so that your HD capable plasma screen actually receives a true HD signal.
We use a LCD projector, and view on an 8 foot wide screen from 13 feet back from the screen. It gives us the real movie experience, but you could get the same "big screen feeling" by sitting 6.5 feet from a 4 foot wide screen since it fills exactly the same proportion of your field of vision as does ours.
We went with the 42 inch because our couch is also close to the TV. It's plenty big enough.
Size does matter but quality matters more, much more.
I have a 40" Sony LCD that has the best picture I have seen yet on any flat panel. I would not trade it for a 60" Plazma..
There used to be a recommended correlation for distance from screen to size of screen. That was for older standard resolution big screens where the closer you sat to the larger screens the worse the picture looked. On the new sets thats not an issue. I have a 61" screen and when I first got it I lived in a 1 br apartment. I couldn't have sat more than 3 or 4 feet way and it looked great. Literally like watching a theater screen at home. Now I have a house and its just right in my large living room. Had I listened and bought smaller it would have been too small.