Bigger screen for sure, but minimize size change at the same time (aka use up the useless white/black area). I can accept a 4" screen. This means a .5" diagonal length increase, translating to about .25" wider maintaining the current aspect ratio. I'd say there's already about a quarter inch of space between the edge of the screen to the edge of the phone if accounting for both sides. So the additional width will really boil down to how thin they can make the edges. Theoretically an edgeless phone should require almost no size increase to achieve a .5" bigger screen.
Oh, and goes without saying, thinner than iPhone 4. Even if it's by 1mm it's good enough for me. But definately needs to be thinner.