When you design the icon, do it at a large size. 1024*1024 is the largest size Apple wants I believe.
I'm going to need to resize it for the different icon sizes that are needed in the app:
50*50, 100*100, 72*72, 144*144, 512*512, 57*57, 114*144 etc.
I know that can have some challenges creating an icon that looks good at multiple resolutions, but do your best.
I would like the icon to look nice on a retina screen, so do try and make sure you take advantage of the additional resolution.
Ideally I'd like to get the original PSD / Illustrator file (or whatever program you use), so I can fiddle with it if I need to in the future.
Make sure you also follow the guidelines apple has set out for icons:
Remember apple applies gloss, rounding etc to the icon, so take that into account. If your icon does not need that applied make sure you let me know.