Here is some additional information:
http://www.faforever.com/forums/viewtopic.php?f=53&t=6089.
The world borders thing is probably true, but I never fiddled around with that. If the map doesn't have too much height difference (the highest and lowest point of a map never have to be apart by more than 20 units - then you can still make a map that works in terms of cliffs, hills, water etc. and it'll also still look good). So that's what I usually do: make map with intial elevation 20, the water starts at 15 and the lowest point is 10 (if I have water). The highest point goes to 30 and if I want mountains, then it'll be 40. How to quickly tone the difference in elevation down is in that link (my first reply there).
Also: if you want to upload to FAF,
just go there, go to the vault and upload it, I don't know anymore... I think you can just ask in that, or it should be somewhere in the sticky posts in this subforum. Please be aware that while there are a load of different maps there, only a few of them will look really good. If you need inspiration on how to make your map good looking, you have to find these rarer maps (Try searching for "loki" or "desert planet" for example). Making a playable map is a matter of hours (and it already includes not making the height differences too extreme, that's a mistake made by almost everyone), but making a real good map will take some (a lot of) patience.
Good luck!