It looks like when I shoot video with
If you need more customization/power on iOS than you get wish the higher level objects, such as
UIImagePickerController, it is recommended to work at the next lower level:
AV Foundation Framework. Apple has some excellent documentation on AV Foundation programming that should come in handy for that purpose.
Unfortunately, even there you are limited to capturing at 640x480 if you do want it standard across all devices. There, however, is a great chart available at the same link (but anchors are broken in the docs, so Ctrl+F to "Capturing Still Images") that lists all the resolutions for various devices under certain quality directives.
Your most solid bet, assuming 640x480 is too small, is to work out some sort of scaling algorithm that would allow you to scale according to the overall resolution.