I think you should be sitting like 2-3 times the diagonal measure of the screen size away from the tv. So if your couch is like 150 inches from your tv, you're in the 60 inch tv range. At least I think I heard that
But hey, in my opinion, the optimum size is the biggest you can get!!!
I have suggested making a cardboard mockup of a screen and hanging it on the wall (for a panel TV or front projection) or setting it on a chair a little away from the wall (for a box TV) and then sitting down on the sofa and judging whether the size is right for you.
I've always heard 30 degrees. It looks like THX is between 26 and 36. The reasoning I have heard is that if you go wider your eyes can't take in the whole picture without moving and if you are having to shift your eyes back and forth during a movie they get fatigued.
... at a 15 foot distance this calculator says I can use a 110.7" diagonal screen. And if it were a THX appropriate screen then 134" is called for. That would about to an 11' diagonal screen - which is about 96" or 117" wide, respectively. That amounts to an 8' wide or just under 10' wide screen. On my 15' wall this would leave either 3.5' or 2.5' free on either side. This sounds ideal for covering the wall with a fair balance between screen and total wall area/length.
The more I think about it - I like the idea of a front projector with a screen size of either 110 or 134" diagonal.
Also note that the resolution of the display in question is important. Achieving large viewing angles requires a high resolution display. Sitting at 1x SW will not be satisfying for a lot of people on a digital 720p display for instance, especially with SD sources like DVD.