Well, it's not like you have to get the exact size from the start. Just pick a size that will allow the creation of a few continents and start with that. If for some reason you need more room just edit the map properties to whatever you desire. Then if you need to relocate certain continents just hold the right mouse button to make your selection size the size of the landmass and then click once where you want to relocate it!
I bet you were hoping for a simpler answer! In that case, 200 x 200 would be a pretty good size for a standard Final Fantasy style over world map.