Different operating systems have different standard sizes.
|Windows Vista||16×16, 32×32, 48×48, and 256×256|
|Mac OS X||16×16, 32×32, 128×128, and 512×512
|Linux||16×16, 24x24, 48×48, and 96×96
|iPhone||57x57, 72x72, 114x114, 512x512|
Icon sizes for Windows Vista and Windows 7
It depends on your application, but here are some suggestions from the Microsoft Vista Icons Guide.
Application icons and Control Panel items
A full set should contain 16×16, 32×32, 48×48, and 256×256.
Could be in 16×16, 24×24, and 32×32. For example, Internet Explorer has a customizable setting: you may switch to large icons (24×24) or small (16×16).
Dialogs and wizards
Microsoft suggests no strict guidelines for that. You should use the closest standard size matching your needs, or even introduce your own custom size.
Icon sizes for Mac OS X
Here are the icon sizes from the Apple Human Interface Guidelines.
Applications, folders and documents should include icon resources in these sizes: 16x16, 32x32, 128x128, 512x512 pixels.
Application icons and System Preferens items
A full set should contain 16×16, 32×32, 128×128 and 512×512.
Could be in 16×16, 24×24, and 32×32. For example, Finder has a customizable setting: you may switch to large icons (24×24) or small (16×16).
Icon sizes for Linux
Here are the icon sizes from the GNOME specification. Bold denotes the primary size for this kind of icon.
|Object and document
||24x24, 48x48, and 96x96|
Icon sizes for iOS (iPhone, iPad and iPod touch)
|Description||Size for iPhone and iPod touch||Size for iPad|
|Application icon (required)||57x57
|App Store icon (required)||512x512||512x512|
|Small icon (recommended)||29x29
|50x50 for Spotlight search results
29x29 for Settings
What icon format should I use?
It depends on the platform you develop for.
|Mac OS X||ICNS|