I dont think there is a "auto resize map" Funktion hidden somewhere
You can export the Highmap and the Stratums, resize/shrink them and import them into a new map with the new, correct size.
But probably it will need some love later as this perhaps creats Bugs in the map (Areas arent passable anymore, for example) as well as it might look ugly at some points.
The Props and the Markers need to be done manually.