I've been meaning to link to this for a few days. Great article by John Gruber that is definitely worth a read. His ultimate prediction:

4.7-inch iPhone: 1134x750 resolution, 326 PPI
5.5-inch iPhone: 2208x1242 resolution, 461 PPI

He admits that if he's gotten anything wrong in his predictions that it might be the 5.5-inch resolution, but he's put so much effort into it that I can't argue with him.

I didn't even think that the 5.5-inch iPhone would see the light of day this year a few weeks ago, but finally the supply chain leaks are starting to come out. It seems that it's now more or less accepted that we'll see both a 4.7-inch and 5.5-inch iPhone on September 9th. If Gruber's 5.5-inch screen resolution proves true, I'll have a lot harder decision than I thought I was going to have. I was sure I'd go for the 4.7-inch variant, but if there's such a huge difference between the screens—and maybe other specs—I might have to give the 5.5-inch phone a good look. It's possible that they could position it as the higher end device. The rumours also suggest that it will sell for $100 more than the 4.7-inch version.