How would you arm or feed the levy? If they are being soldiers you would run out of food. Plus I doubt anyone would have weapons and armor for thousands of men to spare.
That'll be part of seperate rules of course. What I gave is only a part of the rules, of course. I'm not making an entire ruleset at the moment.
If I were to do it, I'd use a supply and unit mechanic. For example, you want to draft one unit of militia, which occupies 100 pop and some popularity loss. This gives you:
1 Militia [Attack:1 Def:1 HP: X/X Morale: X].
You don't have to care about the food for your army, since the pop is still counted as being in the city, and therefore being taken care of. Weapons are paid for at start(and define attack and defense), and you might be able to implement maintenance costs as a balancing factor too.
When the army gets on the move, things get interesting. As long as there's a route towards the city they originated from, they'd get their supplies from there. (Maybe a cost for transport). When supply is incertain, supply caravans are spawned at the city, and need to be moved to the army. Besides, calculating from selfsufficience to numbers isn't that hard.
Name:
Population: 5000 (3000 Useable)
Pop efficiency: 40% needed(Ie, 60% of the pop is availble for jobs)
Garrison:
2 Units of town guard (Attack:2 Defense:3 Hp: 20/20 Morale: 10) (2 maintenance)
5 Units of militia [Only gathered in case of a siege]
Buildings:
-Walls
-Granary (Holds a 3 month food supply.)
-.... (whatever you want here)
Villages:
-2 Farming villages
-1 Mining village
Population usage distribution:
200 city guard
2000 Civilian economy (gives gold in taxes)
400 Supply caravan(upkeep for a 400 man army someway far away)
200 Forges
200 Construction of Improved fortifications