Realistically, Apple could only squeeze in a 3.7" screen into the current form (see pic). They'll have to have some edge space for light capturing. In the image to the left, the most you could squeeze in to the current space is 3.97" and that's with NO edge space (won't happen).
Most Apple can do with the current size phone is 3.7" and that's hardly worth the bother.
Most Apple can do with the current size phone is 3.7" and that's hardly worth the bother.