Take a 4.7" screen and superimpose it over an iPhone SE. It fits.What? Have you looked at the size of the bezel?
If, as rumoured, Apple is removing the space for the home button below the screen and incorporating it via 3D Touch into the screen itself, you can eliminate most of the chin and the forehead while stretching the screen to the edges on each side.
Voila: you have an iPhone about the size of the iPhone SE with a screen the size of the iPhone 7's. No need for 2 different phones.