I have a 50" Panasonic plasma I watch from about 12 feet. Its great, if anything I'd go bigger. I say this because I can stand 8 feet away and the picture is still awesome. Closer than that, you can see the pixelation some. The 42" would definitely be too small. Seems to me because of the aspect ratio that it takes about a 42" TV to give you what appears to be as "big" a picture as a 4:3 CRT 32" or so.
When I was reading about Plasma vs LCD 1.5 years ago, the plasmas were said to be the most reliable TV you can buy. The only drawback was the possibility of burn-in, but the later ones, including mine are supposed to resist that problem. By the way, its exactly the same burn-in as you have a regular CRT TV. I never had burn in on one of those, and I don't have it on my plasma so far.
I doubt that plasmas are "on the way out", since they are building 1080p plasma TVs right now. They are coming down in price just like LCD TVs are coming down in price. But then I have not researched the subject in a while. When I bought mine, I was being steered away from DLP TVs on the basis that you have to replace the lamp and that they have other problems. Since then, the DLP technology has greatly improved to where there is no lamp to replace, they look better, and supposedly don't have the reliability issues they did.
LCDs have beautiful pictures as do plasmas. It could be my eyes, but LCDs seem a little jerky to me on sports. I don't know why.
My plasma is 720p. I have blu-ray and it is awesome even on the 720p. I stood at Fryes one day and watched the same blu-ray movie being played back on Sony 42" LCDs. One a 720p and the other several hundred $$ more, a 1080p. I stood there for at least 10 minutes looking back and forth at the movie pirates of the carribean. As hard as I tried I could not detect a difference between the two. Some with more acute eyesight might can, but not me.
And all you asked about was size.