SP mode allows only 2X unit size while MP 1X. Since the size is determined by DMN script you must have vanilla unit size option to default. If for any reason you do not have the right unit size in game then probably you have messed vanilla unit size option. To fix it:
1. Start DMN select Restore Vanilla.
2. Start a vanilla game normally (not DMN+vanilla).
3. Make sure that your starting armies have the default size (e.g. infantry 160, etc.). If it is not then adjust it through game's Options, Graphics (default is set to large).
4. Exit and start DMN.