As far as I understand it, there is a minimum unit size of 6. Maximum is 240. This is hard-coded, and takes the form of a 'base' size which is multiplied depending on whether you want large, huge etc, unit sizes.

This is hard-coded and not changeable.

This might force you to use siege units as a base for armoured vehicles, since they seem to be the only unit that isn't bound by this.