It seems like for many people the answer is 'Whatever size Apple decides'. For others who have used larger screens, the answer is USUALLY the bigger the better. I like the size of the EVO but higher resolutions would be great. When I use friend's iPhones I hate the size of it. Way too small. Obviously at some point, probably approaching 5 inches, the size of the screen becomes too large to be practical to carry around normally.
One thing I would bet money on though, the size will trend upward even from Apple.
I own the iPhone 4 and IMO I think its one of the best smartphones available, but I prefer/want a 4.3" screen. How do you explain that?