Apple CEO Tim Cook said during Apple's earning call yesterday that "the company believes the iPhone 5 has the 'right' screen size." This does not mean that we will not see an iPhone with a larger display in the future, but that it is not going to be in 2013. And there are several practical technological reasons for that. Unlike Android, iOS and the apps built for it do not automatically scale to different resolutions.
A larger display will require a higher resolution. Apple could place is current 640 x 1136 pixel resolution display in a 4.3-inch display, which would be marginally larger than its current model. This would not really challenge the larger Android's a larger iPhone would be targeting, but simply divide its current market into two groups. This would also result in pitting the a 4.3-inch iPhone with 4.3-inch Android with higher 720 x 1280 resolution displays.
Placing the current 640 x 1136 pixel display on a 4.5-inch or larger display would be even a worse idea. It would destroy the "retina" marketing tool Apple has been using to push the iPhone. And it would pit the iPhone against 4.7 to 5-inch Androids with 1080 x 1920 pixel displays and 4.5-inch Androids with 720 x 1280 pixel resolution displays.
The net effect of a larger display with the current 640 x 1136 pixel resolution will be to make the big iPhone look like a technologically inferior product.
A second area of concern is the developers. With the release of the Apple iPhone 5, with the new 640 x 1136 pixel resolution, app developers are still porting their apps to accommodate the new taller screens. Older apps, designed for the 640 x 960 pixel display, will display black boundaries on the top and bottom, or left and right edges, of the display.
A higher resolution iPhone, to keep app compatibility in check would have to have a whopping 1280 x 2272 pixel resolution. This would mean that app developers, who would have just finished porting their apps for the iPhone 5, would have to again port their apps for the newer larger iPhone.
I have no doubt that we will see a bigger iPhone one day, but before that we are likely to see a newer version of iOS which would have better support for multiple resolutions.