It seems like the whole internet is talking about an iPhone with a bigger screen. John Gruber and Marco Arment said there would be two possibilities. Bigger pixels or more pixels. Bigger pixels would indicate the same amount of content, just bigger. That would not seem like a worthwhile endeavor to me. So here I'll analyze the more content on screen possibility.
More content = more icons
There is one place in the system where there are clear dimensions. This is the organization of the home screen. It is a grid of six by four icons. They are each 120 x 120 pixels with 32 pixels in between and on the outside of the grid. This gets us a 640 x 1136 screen. Adding an extra row and line of icons adds 120 +32 = 152 pixels in both directions. This will get us 792 x 1288 screen.
Pixels to size
If (big if) the pixel size stays the same (326 ppi) that means the screen will be 2,43" x 3,95" and give us a diagonal of 4,64" and a ratio of 1:1,63. I've asked my brother Ysbrand to mock me up an iPhone with the bigger screen. On the left is an original iPhone 5s, and on the right you see a version with the bigger screen (click for a bigger picture).
The phone on the right looks big but manageable to me. I would not be surprised if this turns out to be the solution that Apple uses, but something totally different wouldn't surprise me either. What do you think?