Not sure how I missed this one, but MacAddict published a hack to change the maximum desktop icon size - check it out; they have a pretty funny screen shot! So if 128 pixels simply isn't big enough for you, edit:

/Users/username/Library/Preferences/com.apple.finder.plist

Find the DesktopView Options section, and then the IconSize key; below that is a single number; replace it with something much larger and save your changes. Logout/login to see the effects ... the MacAddict article has much greater detail, so check it out!