ios sizes

iPhone3:- 
Screen resolution: 72 ppi
Screen size: 320 x 480 px
Icon size: 57 x 57 px
File format: PNG-24

iPhone4:-
Screen resolution: 72 ppi
Canvas size: 640 x 960 px
Icon size: 114 x 114 px
File format: PNG-24


iPad:-
Screen resolution: 72 ppi
Canvas size: 768 x 1024 px
Icon size: 72 x 72 px
File format: PNG-24

Reference:
http://mobile.tutsplus.com/tutorials/mobile-design-tutorials/iphone-design-templates/