In essence, there are only two screen sizes to mess with when doing iPhone/iPad development. In the Android world, there are many many different screen sizes and resolutions. There, I have solved a number of interesting but time-consuming puzzles with combinations of -ldpi/mdpi/hdpi drawables (and dimens) and small/normal/large layouts to make sure things look ok across the spectrum of possible devices.
As of iPhone 4.4S, Apple did have to introduce a new screen resolution for the phone, which resulted in them introducing the notion of an Android-like device-independent pixel called a "point". The size of the phone's screen is 320x480 points no matter what.
So, here is the blissfully small list of possibilities:
|Possible Screen Sizes for iPhone, Pad|
|Device||Screen Size in Points (Pixels)||Resolution|
|iPhone 3 and prior||320x480 (320x480)||163ppi|
|iPhone 4, 4S||320x480 (640x960)||326ppi|