I think like you keep it the same size as a normal icon, but like...lets put it this way..
the icon is supposed to be 48 x 48 (example), but you want it to be 24 x 24. So you basically make the icon size starting with 48 x 48 but you draw your image 24 x 24, and keep the outside transparent, so that it LOOKS like its 24 x 24. get it? Idk lol...don't take my word for it.