While we have no real idea what a new iPhone would really look like, the folks over at MacRumors have created a cool paper model to get an idea of what a 5-inch version would feel like to hold. The model comes as a downloadable PDF, which you can then print and assemble.
In our testing, we felt that the larger iPhone fits surprisingly well in our hands. However, the ability to operate the device with one hand is significantly compromised compared to the iPhone 5. Whereas the iPhone 5 requires very little stretching to reach the opposite top corner of the screen with our thumbs, the 5-inch screen would definitely be a two-handed device for most apps.
Other example screens are also available, so you can get an idea of what apps would look like. I’m definitely going to try and assemble one of these.