It is possible to dramatically increase the Finder's icon size, beyond the official 128x128 pixels. I tried it for the Desktop with the following command in Terminal:
defaults write com.apple.finder DesktopViewOptions -dict IconSize -integer 256; killall Finder
You can use any value you want, up to a maximum of 512. Warning! You might see some slowdowns on older machines, at least in OS X 10.5. This is due to the fact that drawing the high-resolution 512x512 icons seems to be hard work for some graphic cards and/or G4 processors.
[robg adds: We actually ran this hint a longgg time ago. However, back then it referenced an article on MacAddict's site, which has since vanished. I thought this would be a good chance to update the hint, especially with 10.5's amazing new 512x512 icons (though you can do this in any version of OS X after the public beta). Click the image at right to get the full-size effect: the icons as seen at 512x512 on a 1280x1024 screen. I am not suggesting that this is a usable size, of course!]

