Ideal Size For. Ico

Short answer: 16 x 16 pixels.

Long answer:

.ico files can actually contain multiple images, at multiple colour depths - you can provide 16x16, 32x32, 48x48 and 64x64 in a single file and the OS will pick the best one to show.

Of course to keep the file size low you don't want to put too many in there, but if you think people might be saving a link on their desktop (say it's a web application) then it's probably a good move.

See Microsoft's recommendations.

Basically, 16x16, 32x32, 48x48, and 256x256 for application icons. I guess the window icon is a toolbar icon so 16x16, 24x24 and 32x32.