Marco Arment:
The recently rumored, larger-screened “iPhone Math”, or more likely “iPhone Plus”, is plausible as an additional model (not a replacement) alongside the 4” iPhone. And there’s a good chance that it would have a 4.94”, 16:9 screen.
Arment does the math to detail how the iPhone Plus would theoretically be implemented such that the impact to developers would be minimal.
By keeping the pixel dimensions the same as the iPhone 5, no app changes would be necessary. While the larger screen would hinder one-handed use, two-handed use would actually be easier because the touch targets would all be larger, and UIKit’s standard metrics and controls still work well at that physical size.
Check out Arment’s mockup to see how the phone would fit into Apple’s lineup. It makes sense - visually and technically. Making such a device would be an admission from Apple that there is a market for phablets. If an iPhone Plus is released, I’ll have to eat crow.