Windows *.ico files and Mac *.icns files contain multiple size icons. A lot of the icons you can buy also come in various sizes. But, as far as I'm aware, most implementations of toolbars set the size that's displayed. Are there any guidelines as to the best size to use on different platforms?
I normally use 16x16 since they take up the least space and arguably, quite legible even with very high screen resolutions.
Are there any guidelines as to the best size to use on different platforms?
It's 24x24 and 48x48 in GNOME.
I take a hi-res base, and make an icon with all supported sizes in it...
16x16 -> 128x128 (I can't remember if 256 was an option as well...)
This is because on Windows XP on my tablet, I like my kids to be able to navigate easily, so I have massive icons on the desktop. Looks pretty too. (If I was using toolbars, I'd potentially still do large sizes).
Do what you like, but if you're installing an app on a computer, then I reckon you make every possible size.
PS. I don't usually touch up my smaller sizes - I find the icon programs generally make a reasonable 16x16 icon.
PPS. If you have a toolbar operation that has another screen, then you might want that screen to have a large icon [in the background].