I believe you can add more terrains in the same manner you made your first terrain. (Terrain pallet, create new terrain.)
I would suggest making the new terrain larger, then using the lower terrain tool to hide the terrain under what you have already made. I am not sure if there is an easy way to move terrains as the translate tool doesn't seem to have an effect.
Concerning max map size I'm not sure. Currently I am messing around with a 2000x2700 terrain. Aside from the draw distance being no where large enough to see the entire terrain at once, the only issue I am having is generating an AI grid for the map. It takes a long time, and always crashes near finishing.
Last edited by ITheWestSideI; 17/07/14 03:31 PM.