I was just pointed to this fantastic article concerned with just how large a fantasy army can get, assuming the usual pseudo-middle age setting. It has some good details, I recommend it. I would add to that this website - which I may have linked before - which has a series of articles about putting fantasy settings on a sensible basis.
It must be the strategy gamer in me that keeps me thinking about these aspects so much. While I was writing Twixt Heaven and Hell I often had to remind myself that not a whole lot of readers are likely to be interested in the logistics of how soldiers are supplied and organized - but I was thinking about it. That's why I was delighted to get an email from one of my very earliest readers asking if I had some explanations for possible discrepancies. I was happy to send him a lengthy response on my reasoning for why Bastion can field so many soldiers at once compared to why Pyre developed the transportation magic before their enemy.
This is one reason that part of the extras included in the first Wandering Tale collection will include what some might refer to as "historical matter." Because while it would be dry and somewhat boring to a lot of people, readers who love the world of the Wandering Tale might love to know more about the history and setting. Maybe almost as much as I love to write about it.