Both are right. The problem is: You should never be wanting a map of over 256x256. Those size maps are nearly always bad maps. If you need even more then that much space to make your map in, reconsider your map, because 99% of the time you're doing something very wrong. You don't need that much terrain, since it'll only annoy people. You don't need more fighting space, you don't need more room to show stuff. You just trigger to change the area to what you need if you need more room.
It's your choice ofcourse, but in my opinion (and for as far as I've checked, a bunch more agree) you shouldn't use large size maps. It'll only ruin your map.