If you meant to make your map bigger, then you change that setting in the map maker in the "world" tab, in the "width" text box. but if you really want a bigger height map then your height map and mask can be any size (2^n)+1
so you can use sizes of 257x257, 513x513, 1025x1025 ect
Personally, i only use 1025x1025 for very complex and detailed maps because 513x513 suits most needs of everyone