I might (tiniest voice) get around to it, but just in case I don't, there's an easy way around it. It might be useful for the answerer.
All you need to do is prepare a grid for the overworld, and trace it onto every image of a Secret Base. This would get you the moving space (aka tiles) of each secret base, whichever has the higher number is larger one.
Takes something like 3-5 hours at the very most tbh.