Marco Arment,, suggests that a 4.94-inch 16:9 iPhone+ is plausible and posts some nice mockups of what the device might look like.

What if an iPhone Plus used Retina iPad screens with iPhone 5 resolution, keeping the rest of the design sized like an iPhone 5? Its 640 × 1136, 264 DPI screen would measure 4.94” diagonally, and it would look roughly like this next to an iPhone 5.

via iclarified