I think that would really be a matter of taste. I doubt there's a
golden ratio.
I like small.
I don't normally use desktop icons, but, when I use a desktop that has them (such as Lubuntu on the Wubi install on my Win7 box--I need to have a Windows box, but that doesn't mean I have to use Windows unless I must), I normally reduce the size from the default. On my Wubi, I have the icon size set to 28 px and the text to 8 pt.