You can do it in the mapmaker: go to map in the toolbar and select "Calculate Terrain Tables". There you can change the grid size from the standard 100 to anything you like.
My "Benouville - Ranville Bridges" map has a grid size of 25. For such a small and detailed scenario it had to be done. I've tried to do it with the full "Caen - Ouistreham" map, but due to the bigger scope of the scenario it quickly became to slow while running the game. So beware if you plan on using lots of units in your scenario. Smaller grid size means more calculations to be done when units are moving. Lots of moving units will generate lots of calculations. Put them both together and you've got slowdowns, so be wary and keep checking if it doesn't run to slow.
Ah, I see. Haven't reached that part yet. Thanks. Will experiment a bit with it. Eventually when I finish somewhere down the line, and am satisfied with the map I'll upload it probably. Maybe even with the scenario I have in mind.