Marco Arment, app developer and tech writer, has a good guess at what an iPhone with a larger screen could look like.
There have been several rumors lately that Apple could release an iPhone with a ~5-inch screen as early as this summer. Arment thinks Apple could pull such a device off by trimming down the high-resolution Retina displays used on iPads.
Keep in mind that this is just speculation, but Arment suggests:
The iPad Mini uses iPhone 3GS-density screens at iPad resolution. What if an iPhone Plus used Retina iPad screens with iPhone 5 resolution, keeping the rest of the design sized like an iPhone 5?
The mockup is in line with Apple's design and would allow the company to cram in a larger battery so the phone could theoretically last a full day.
The best part of Arment's concept is that no app changes would be necessary. The screen's resolution would come in at 640 × 1136 with 264 DPI, which would allow the apps to be scaled up to a larger display.