You can know it with the UIScreen
properties
[[UIScreen mainScreen] bounds].size.height;
[[UIScreen mainScreen] bounds].size.width;
for iPhone 5 and iPod touch 5gen the height is 568
for other iPhones and iPods the height is 480
Edit: on iOS 8 the height and the width deppend on the orientation, so this sizes are for portrait, on landscape this sizes will be for the width instead. So, best choice is to read both and do the max