It might be worth giving the
Mitsubishi HC6500 a viewing along with these projectors. So far they're the sharpest projectors we've seen, with excellent black levels to boot.
As for the size of the screen, the normal rule is about 1.5 or 2x the screen width - this can be increased a little for HD, but DVD will look a little ropey. I'm looking at getting a bigger screen than I really need in my room, as most of my viewing is HD nowadays, and I no longer invest in SD.