Langui.net

1. 57 px, iPhone – good ol’ classic.
2. 72 px, iPad
3. 114 px, iPhone 4 – make sure your icon shines on the Retina Display.
4. 512 px, iTunes – Used in iTunes and in the App Store, where it’s sized down to 175 px. Sadly, you can’t provide the 175 px version directly.
5. 29 px, iPhone Settings/Spotlight, iPad Settings – used in these table views. Minor, but still important!
6. 48 px, iPad Spotlight – yup, the iPad uses a different size for Spotlight and Settings. This size is controversial! Apple’s docs actually say the icon is 50 px, but then there’s this note: The final visual size of this icon is 48 x 48 pixels. iPhone OS trims 1 pixel from each side of your artwork and adds a drop shadow. Be sure to take this into account as you design your icon. How weird!
7. 58 px, iPhone 4 Settings/Spotlight – that’s right, you have to make both 57 and 58 px versions of your icon – d’oh! Good luck aligning this if there’s a line running down the middle of the icon.
8. 64 px document icon – who knew: iOS apps can provide document icons. It’s unclear how these will be used – they don’t do much currently – but it’s not a bad idea to start planning now.
9. 320 px document icon – why not 256, darn it?
10. Let’s say Apple comes out with a high-ppi iPad. That will mean at least 2 new sizes – maybe 144 px and 96 px.

From:  All the sizes of iOS app icons (Neven Mrgan’s Tumbl)