I would use a base formula. For example.
Major states get 1 Regiment (9 Companies) per production line.
All states get 1 Company per world
Therefore a large state with 100 worlds and 12 production lines would have 208 Companies, or about 23 regiments, likely deployed as Battlaions, as this seems like a Mad Max setting.
A small 10 world state would have just over a Regiment.
From there, you take the base and modifiy it to suit the needs of the story. However, if you plan to make a game out of it, this does balalnce forces.
But that's just me.