changing it's size vertically is easy - just add or remove a few icons
changing its size horizontally is only achieved by editting the skin in skinstudio and possibly the graphics in a suitable program as far as I know (or switching to a different-sized-start-menu substyle). Open up a few existing skins with varying sizes of start menu in skinstudio to see how it's done.
If you have a more detailed question I can try and fix you up with a more detailed answer
