You can change the size of the Desktop Icons with your mouse wheel. When you right click on the Windows Desktop there's an option to change the size of the Icons from Medium, large or the Classic size.
If you don't want to limit yourself to these default sizes and you want some extra you could check out this.
|Desktop with Small Icons|
|Desktop Icons Grew Large|
This is a Vista Specific Trick. Feeling Lucky?
Left-Click anywhere on the Desktop, hold the Ctrl key and move the Mouse Wheel. The Desktop icons will Shrink or Grow accordingly.
Uncheck the "Auto Arrange" Option while performing this trick for more interesting results.