Just in case there weren't already enough signs pointing towards the launch of a larger iPhone or two later this year, a new Adaptive Display framework in iOS 8 is a telltale clue that wider and taller screens could be on the horizon. As reported by The Guardian, the developer protocol is called UIAdaptivePresentationControllerDelegate, and works with a presentation controller to determine how to respond to size changes in apps.
If that sounds like a bunch of garble, allow me to explain in slightly more simplified terms. Apple has improved two frameworks called UIKit and AppKit, making it easier for developers to support multiple screen sizes and orientations. Apple does have multiple screen sizes, ranging from 3.5-inch and 4-inch iPhones to 7.9-inch and 9.7-inch iPads, and the consensus is that 4.7-inch and 5.5-inch iPhones will soon crack the list.
This screenshot on Apple's WWDC sessions page could be a subtle hint…