Going by the landmass of Daggerfall, Illiac Bay is roughly the size of Great Britain, right? Since Illiac Bay comprises both Hammerfall and High Rock than High Rock itself should be the size of England plus Scotland. (Correct me if I'm wrong.)
The comparison is adequate because High Rock has both rich low lands throughout the peninsula, but it also has the less populated highlands in the eastern mountain range (which suits Scotland I guess).
The number of men a province can muster would depend on:
1. The wealth of a region (and how it would maintain a standing army)
For comparison, England during Harold Godwinson's time could muster arround 10.000 men (Battle of Hastings). Two hundred years later, England showed up with 25.000 at the Battle of Bannockburn. Advances in technology, commerce and agriculture allowed England to become a much richer country than it did during the 11th century.
Imperial rule would've secured trade routes and provide prosperity for the region of High Rock, but by the time of Skyrim, that prosperity ought to have reverted back a bit. High Rock seemed completly divided in Daggerfall, each noble looking after his own interest. The Empire did get involved every now and then to pacify the disputes. A house that's divided is much weaker than a house that's united. However if constant civil war is reality, then the militancy and fighting traditions would be higher than in a united kingdom. Think of the Swiss.
2. The population of the region (and the militancy of its people)
How many settlements were there in Daggerfall?
Let's say there's 4 types of lands. Taking the size of England as an example, England is a (Good) land with a high density, Bangladesh, which is about the size of England is an (AWESOME) land with huge density. Greece is also the size of England and it's a (Okay) land with medium density, Oman on the other hand is a (Bad) land with low density.
Which would you reckon are the more prosperous lands in the Empire? Not Provinces as a whole, but parts of provinces (i.e. Nibenay Valley vs. Colovian Highlands)
3. It's leadership and society.
This is very important. Tribal societies (I'm guessing Black Marsh comes close to this) have a much higher militancy than feudal societies. Feudal societies are build around the warrior caste, so military services is only reserved for the high born, while the peasants are often reluctant to take up arms. Tribal societies teach each man from a early age to use weapons (to hunt or protect the village), most men in a tribe would know how to fight (and they'd be pretty good at it too, think of the Mongols or the Seljuk Turks).
The Imperial method though, would be cross between the too. Warriors aren't a separate caste, each can join the ranks, but there are probably more farmers in that society than soldiers.
The Royal Houses of Morrowind functioned around this basis too. Everyone had membership, provided he was loyal to that house alone. They were also based on blood ties and had some similarities to the Ashkan tribes in Vvardenfell.
There's also slave armies that House Dres could build up. You take the children born in slavery and raise them as fighting men from a young age. They're more loyal than mercenaries, but they can still turn on you as it happened with the Egyptian Mameluks during the Crusades. The size of a slave army depends heavily on the size of your treasury. Saladin had slightly more men than the Crusaders, but the more wealthy Ottoman Turkish armies were almost twice the size of anything the Europeans could throw at them.
Care to speculate about this? I think there were some books that actually gave concrete numbers of the size of armies during historic battles. But I haven't played or read anything from the game in such a long time.