Look into the map Solar Conquest for WC3, it may give you some ideas on how to do what you're asking. Also, if you want a bigger map size, going to its border properties and increasing playable boundaries will extend the maps bounds to its maximum size, but beyond 256x256, no, thats what you'll have to work with. You can decrease the size of all your units and buildings and zoom in the camera appropriately to make the map seem bigger, so that could work.