The largest size included in the Vista icons resource is 256 x 256 pixels. This may sound quite large for our present 96 DPI displays, but it will be quite suitable for high-DPI displays being developed which will support 240/320 DPI resolutions. Including the 256 x 256 pixel size in the Vista icon resource guarantees that the icons will display at their highest quality on these larger monitors.
It's very easy to change the size of icons in Vista. To bring back the Classic Icon sizes do this trick from OCModShop.