ww2commander
Emperor
Am I missing some big point about SDK?
Different sizes of the same icon are used in various areas of the game and you need to create each of these sizes depending on what your modding. There is a little text file that comes with the templates that tells you which template sizes an element (i.e. unit icon, civ icon) needs to have.
Easiest way is to use the largest 256x256 template and then resize it to the other sizes as Kael suggests. Also, always remember to set VFS to true under the properties of each image file you import into ModBuddy for any mods your creating.