You want to think in terms of aspect ratio first, and pixels second. Currently (and this may change) the iPad has the same aspect ratio that's shared between an

You want to think in terms of aspect ratio first, and pixels second. Currently (and this may change) the iPad has the same aspect ratio that's shared between an iPad, an iPad with Retina and an iPad mini. On the other hand the iPhone 5 and say the iPhone 4s have different aspect ratios since the iPhone5 is taller. Also don't forget that your design has to be responsive so that it can go from horizontal to vertical. And then keep in mind that if at some later point you want to port to Android that there really aren't any fixed aspect ratios since there are so many devices. My advice is that you should sketch and then use an app like Keynote to see what your interface looks like on the device. You can be shocked sometimes just how different things feel when you go from a personal computer to a mobile device.