Standard icon sizes are 16x16, 32x32 and 48x48 with 16, 256 and 16.7 million colurs. At a minimum you have 16x16 and 32x32 @ 16 colours, usually you have the 256-colour version of those and the 48x48 size, and under XP and later you get the 16.7 million colour versions (as those do alpha transparency).

The 16x16 gets used as the window icon and when viewing as small icons, list or details, the 32x32 in the alt-tab list and when viewing as large icons, and the 48x48 when viewing as large icons (with large icons switched on in display properties), and in the tiles view.