Rene Ritchie at iMore imagining how a larger iPhone would work.
If Apple were to make a larger iPhone, I would guess a 4.8-inch screen with a 272ppi. I'm still not convinced though, I see no advantage over the existing iPhone 5 that makes the added bulk and weight worthwhile. But as Rene suggests:
If people are buying massive amounts of premium large-sized handsets, Apple could one day decide they'd rather those people buy premium Apple-made large-sized handsets than competing brands. If Apple sees value in increasing the scope of their addressable market, and thinks it will help them make a better product and sell more units, then that's why Apple would add a bigger iPhone to the mix.