DVD 480p
RLA, standard DVD's are encoded in 480i and then upconverted inside the DVD to 480p. Depending on how good or lousy the upconverter is you'll either get a good or bad picture. I fully believe some consumers prefer SD at 480p over poorly upconverted 480i. Even high bit-rate DVD's are encoded at 480i. You'll have to wait for Blu or HD-DVD before you'll see something improve there.
The prefered screen size is really a personal choice no matter what THX says. I'm still waiting for the day they'll say you don't need to upgrade your rig when something newer and "better" comes along.
In my opinion the best screen size depends on the type of content you typically watch. If it's replaying old VCR tapes, don't even try to go for larger screen sizes. I always compare that to looking at a low-res photograph with magnifying glasses. If all you watch is HD, fine, go for the biggest you can get.
