As already mentioned, each icon set can contain one or two distinct groups of icons. For simplicity, these are referred to as the small size and the large size. However, you are not limited to only two sizes of icon-only two groups of icons. It is possible to have different icons of multiple sizes within the one group. For example,
<set size="small" width="20" height="20" filename="MyIconsSmall.png">
<icon name="copy" row="1" col="1" />
</set>
<set size="small" width="24" height="24" filename="MyIconsMedium.png">
<icon name="move" row="1" col="1" />
</set>
Note that both set nodes above have the size attribute set to small This example would add two icons to the small group – one, called “copy” that is 20x20 in size, and one called “move” that is 24x24 in size. Even though they are different physical sizes, they would both appear together in the small group for this icon set. The icons you provide in the small and large groups do not have to correspond – you do not need to provide both small and large forms of every icon (although it is obviously better for the user if you do.)