The iPhone Air Mini has a 3.5-inch display with the Apple A7 processor and 2GB of RAM under the hood, and powered by a 2,000 mAh battery. The iPhone Air has a 4.7-inch display powered by an Apple A8 processor with 3GB of RAM with a 2,500 mAh battery keeping everything switched on. The iPhone Air Pro goes up to 5.5-inches with the same Apple A8 processor but with 4GB of RAM and a 3,000 mAh battery.
Here is a pretty cool concept video with three versions of the next “iPhone” - the iPhone Air mini, iPhone Air and the iPhone Air Pro (shouldn't that be just iPhone Pro?). Looks pretty cool, but of course this is just a dream that some render artist has come up with.