FIRST Backup original file-----------------------
Go to

X[your steam file location]:\Steam\steamapps\common\Going Medieval\Going Medieval_Data\StreamingAssets\MapV2
Find
 MapSizes.json

Copy it to a safe place. Just in case.


Mod Installation-----------------------
Download my mod and extract the  MapSizes.json inside. 

Replace the original file in: 

X[your steam file location]:\Steam\steamapps\common\Going Medieval\Going Medieval_Data\StreamingAssets\MapV2
with new MapSizes.json you just extracted.

Start your game. Enjoy it.