Note the similarity between the icon and the rendering of the phone seen on This is my next. Using the Photo Stream icon as a guide, it would appear that the new Apple iPhone 5 will have a 3.7 inch screen. Of course, there is the chance that the icon was developed without any regard to the next-gen iPhone. The good news is, with an expected introduction of the model later this month by Apple, we won't have that long to find out how accurate the Photo Stream icon is to the actual Apple iPhone 5.
The icon from the Photo Stream beta (L) resembles this rendering of the Apple iPhone 5 (C); based on the icon, the Apple iPhone 5 would have a 3.7 inch screen (R)