YAY!
I found out how to make the map larger and how to edit it.
BUT:
There are some strange things:
make the map on x-widh is 12 and the y-widh is more than 50
(Don't forget to create the map!!)
If you go down, there won't be any blocks any more, there'll just be many slimes.
Try it out!!
I don't know why, but I think it's a little bug (whitch can be ignored if you don't make so large maps)