Making a map bigger than 513x513?

[cytofin]

Is there any way to make a height map using Gimp2 bigger? I have seen so many beautiful maps and was wondering
 how do the creators get them to be so big. Gimp only allows me to get to 513x513 or it will crash in-game. Is there some type of special art program or some type of software? If so, please comment because I am interested into getting into map making and have a lot of trouble when it comes to getting a bigger map. if that is in any way possible, since I have seen so many created maps that are just humongous and I doubt they are 513x513 just looking and exploring them without going into any portals. 

[greenart6]

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: