A designer is making some Windows icons (.ico) for us and I want to make sure the various sizes (16px, 24px, etc.) will look Ok when Windows displays them.
I created a Test icon with a # for each size (16 for the 16x16 px version,etc.)
From my tests, Windows-7 uses:
32 x 32 : Start menu (popup menu) 16 x 16 : Start / All Programs menu 256 x 256 : Desktop menu
Where would Windows use the 24x24?
16 pixel icons are used by Explorer in List and Details views, in title bars, in the notification area, and in the classic taskbar.
24 pixel icons aren't usually used, although they are used by Explorer in certain zoom levels.
Note that non-standard DPIs will use different icon sizes everywhere.