The size of the heightmap doesn't affect the size of the actual map ^^ In-game under the second tab in Map Maker, there should be an option like "Width" where you can put in how large you want the map to be on all sides, as well as the height of the map. I don't believe you can make heightmaps a different size other than 513x513 (there might be one other size that works, but this still doesn't effect the size in-game)
Hope this clears things up c: