WHICH REMINDS ME!!!
Something that we probably should have done at the start was to decide how many soldiers each hold has at it's command, and then how many soldiers are in the armies of the Legion and Stormcloaks. This kept me up last night while I tried to figure it out, and this is as best as I could think of to make it balanced yet realistic.
The number of soldiers (guardsmen) in each hold would vary accordingly. This is also made under the assumption that the cities themselves are much bigger than they are in game, and that there are more smaller settlements (eg, Riverwood, Shor's Stone) in each hold than in the game. Each hold's capacity for troops would depend on their climate, size, number of settlements, topography, and any other clues that were given in game. (Note, this is soley for a 'paid' garrison. This doesn't cont militia levies raised when needed, such as being done in Whiterun at this very moment).
Haafingar (Solitude);
- Largely mountainous but a very strong, well defended city. One small settlement (we can take that as meaning maybe two or three in the RP). Plus, being the capitol, it has access to a large population and sea trade. It's also the base of Legion Operations, so it's standing garrison of guardsmen doesn't need to be very large. That said, I think that it's own Hold Garrison would number maybe 1000 men, due to it's status as both head of the Legion, Capitol of Skyrim, and Home of the Last High King.
Hjalmarch (Morthal);
- Swamps, wetlands and marshes, small city, no small settlements in game (maybe one decent town, or a few tiny villages). Unlikely that it could muster a very large standing force of guardsman. Perhaps an army of 500 at best.
The Pale (Dawnstar);
- very cold, large forests, mostly uninhabited, though covers a large area. Once again, the city is small and there aren't town sized settlements within. Once again, probably a garrison of 500.
Winterhold (Obvious);
- A city in decline, very few residents, and the inherit distrust for the College. Most likely has a very small population and is so out of the way that very few men would guard the hold. Perhaps 300 soldiers.
Eastmarch (Windhelm);
- A very large and well fortified city, the old capitol of Skyrim and centre of Stormcloak operations. Once again, like Haafingar, there is one town sized settlement (scaling up to two or three). In fact, it is very similar in many regards to those found in Haafingar, so we can assume then that it would have a similar sized garrison. Seeing as the guardsman are also Stormcloaks, the home garrison could be 1000 as well (got to make it fair), but Ulfric would no doubt want more men in the fight so in actual fact, the garrison could be 700.
The Rift (Riften);
- A decent sized hold, also possessing a fairly large city. Lush habitat, good climate, and two settlements in game (translating to four or five towns in the RP). With a decent population and favourable conditions, the Rift's Hold Garrison could probably stand at around 800 men.
Falkreath (obvious);
- A medium sized city, one in game settlement (which is destroyed), so probably a few villages. Largely forests and hills with mountains. Probably capable of producing a decent sized force, the Hold's ability to summon troops is reduced. Where once it could have held around 600 men, it's hold garrison would most likely stand around 400.
The Reach (Markarth);
- A very mountainous hold, so the likelihood of a large population to draw troops from is unlikely. Most of the population would live in Markarth, which would stand as a sizable city. However, due to the two cultures living in the Reach (Nords and Native Reachmen), the chances of a large, united army, as seen with Solitude, Riften and Windhelm is unlikely. With one settlement in game, a low population and a divided culture, this hold could probably only produce a small army. We are also told that without the Legion, they were unable to hold back the Forsworn. The garrison force of the Reach would probably stand at around 600 men, which could easily hold the Dwemer made city. Further, with the silver mines, these men can be paid and armed well.
Whiterun (This is going to seem really bias)
-A large city, with mostly open plains, ideal climate and two in game settlements (probably four in the RP). Vast open space ideal for farming, and plenty of game to hunt. It's central location makes it ideal for trade, and probably boasts a decent population as well. I think it's also the largest Hold in Skyrim. (Ideal place to train cavalry?). It is said to have had the potential to become the "Skyrim equivalent of the Imperial City". I can easily see this hold's standing garrison of guardsmen numbering around 1000 (In a sense, balancing the capitols for Legion, Stormcloak, and Neutral capitols).
Legion
- Seeing as the Legion is based on the Romans, and Tullius has the 4th Legion at his command (correct me if that last statement is wrong), then I would wager that the Imperial Legion army in Skyrim would be around 5000 men thereabouts, once you factor in local recruitment and the lack of reinforcements to replace lost troops. Obviously, they would be divided in Cohorts (and even smaller centuries, as can be seen by the Pyrrhic attack on Fort Hraggstad), so the largest army I imagine the Legion could gather in one place without under-manning garrisons would be around 2000 - 2500.
Stormcloaks
- As the war seems to be fairly even handed, the Stormcloak army would be similar. I imagine it standing at around 6000 men. Remember though, Stormcloak soldiers also double as Eastmarch Hold Guardsman, so in reality, the actual army would stand somewhere between 5000 to 5300 men, depending on how many men Ulfric is willing to move from defending Eastmarch to the front lines. The largest force they could muster would be similar to the Legion, though perhaps with the advantage of a hundred or so more men.
Other factions of interest
-The Reachmen army, if it could be called such, is scattered across the hold in most of the redoubts and fortresses. Seeing as each practically every member of the Forsworn doubles as some kind of fighter, they might number around 500 - 600, though seldom would they gather in such a large force, more likely in small groups numbering little more than 50, though on occasion, perhaps as many as 200.
Feel free to refute any of this of course