The iPhone's display is 320x480, so assuming it's really 2x/4x, that makes the display resolution of a "doubled" app be 640x960. The iPad's display is 1024x768. So that's an excess of 32 pixels on each short side and 64 pixels on each long side.

It's a 132ppi display, so 32px is just under ¼" and 64px is just under ½".