Windows Vista has many new icon sizes.  Using a simple mouse scroll wheel trick that was primarily used in Internet Explorer to change the text size for years, it is now possible to change the icon sizes of almost any window you are in, including the desktop.

While on your desktop, hold down the CTRL key and scroll your mouse up for larger and down for smaller icons.  You can make your icons very small or extremely large as shown in the two examples below.

Large Icons

Small Icons