I'd say a 15.4 inch screen like the midsize macbook pro, again i think the hovering finger tracking tech that Apple's working on will go a long way in facilitating a brand new touch based OS that more closely resembles OSX than iPhone OS. Finger tracking technology that feels precise like waving a stylus across a Wacom for mousing around and then clicking when you touch would go a long way toward making this a reality. A touch based laptop replacement is inevitable from Apple and will surely make traditional laptops more of a niche, though when this actually happens is debatable. Apple can certainly cash in on the appstore through iPhone OS for a while before the competition catches up.