The question is first and foremost to select the right size of the theater of operations you want to cover. If you take a map scale equivalent to that of BOA, our original bitmap is 8000 pixels wide for 9000 height, which is a big map...
Yours, based on the previous map sketch you showed, could be something like 9000 width to 8000 height (reverse to BOA)
My advice is first you do a map sketch in Illustrator format (any size, it can be resized easily later on), with all regions names, info on terrain types, cities, borders, etc...
From this sketch a model can be done, and once there, we shall provide you later on with the specific tool we used to "cut" the map into the components needed for the engine
A lot of work to do first... good luck