osterknact, pm me your e-mail, and I'll send you an army list I wrote for Stirland the other day.
I see we think similarly on some points. Perhaps the army list helps you to flesh out Stirland.
As for the advantages and disadvantages of a large population... I think we should approach the question theoretically, with some historical anecdotes. Here are some things to consider:
1. Every pre-industrial state wanted to have large populations, especially because of recruits for their armies. In this sense, small population is only a disadvantage. There's nothing to gain from a small population. (Anecdote: when a 17th century Swedish king finally had a very thorough census in front of him, he was superbly dissappointed because of the small population of his realm. In addition, the census was kept secret, because the Swedish were afraid their enemies would attack them knowing how weak the realm was.)
2. However, there's a limit to population: food. If the population is too large to be fed, people either need to migrate or starve to death. Usually, the adventurous poor migrate and the non-adventurous poor die. The elites, the middle classes, and the well-off farmers have the judicial system to protect their possessions, and will grab arms (ar call the military) to protect them, if the poor try to eat what is not righfully theirs. Popular unrest is a bad thing for any state. Thus grain supply became early on one of the main concerns of pre-industrial rulers. (Which doesn't mean they did any good to secure it.)
Now, we need to decide if the Empire is approaching its limits of population, or if the Empire perhaps still has room within the provincial borders to found new villages and colonise virgin lands. The existence of the vast forests makes it quite clear the Empire could do the latter. The existence of crowded cities suggests that at least in some regions people don't think colonisation could give them better life. This is of course the result of the chaos and greenskins etc. in the forests. The balance of danger is different than in early modern Europe. We need to ask, how much must be invested in a colony so that it will become secure and start producing something for the greater good (i.e. men can be enlisted and taken away from the colony without the colony becoming so weak that beastmen and goblins destroy it).
This, I believe, might hint to an altogether different effect the population might have on enlistment. Here are the options:
Colonists: the state is actively tied into a colonisation project, and this is reflected in the enlisting options. The new colonists need protection, and cannot be marched away from their homes without endangering the whole prospect of the colonisation project.
Advantage: Local Militia. The army gets one Militia Command Group for free. However, it must include at least one Militia contingent that is not a detachment, but a real independent unit.
Disadvantage: Dispersed Garrisons. The State Troops have a maximum unit size, perhaps 30. (Although this might be eased a bit by stating that one State Troop contingent has no limits.)
Urban Poor: the state is not taking care of its excess population, and so the poor gather in the cities. This opens good prospects for enlistment, but on the other hand, the cities need to be heavily policed also in wartime, tying up trained men.
Advantage/disadvantage: Cheap Militia. The army may buy Militia at a cost of 4 pts per model. (CG still costs normal.) However, the militia can never operate as detachments.
Disadvantage: Limited Detachments. The army may include max. one detachment per one unit of State Troops. This means that a State Troop contingent may still have two detachments, but for every State Troop contingent with two detachments, there must be a State Troop contingent with no detachments at all. (They're policing the cities.)
Of course, the third option is none of the above, in which case the province plays along the normal rules.
As of which province is in which state, I don't know. I guess Reikland could have Urban Poor while Middenheim/land has colonists and e.g. Stirland has no advantages or disadvantages in this respect (census).
I think this would be a fluffy way to incorporate some familiar themes from RW history.
-Z