I agree with Peter, my iPhone is small and I use OzRunways on it often, just perfect for a bloke who can't be bothered with the clutter of an iPad in the cockpit. Sure the tiny screen doesn't show much real estate but it is still fine and it is easy to read the ERSA etc just be zooming on where you need to.
CASA do specify a minimum screen size I think if it is your sole means of getting info. Mine is not but it is all I actually use in flight.