Fantasy author looking for tips on army sizes

[Midgardsormr]

300,000 would be an enormous city by Middle Ages European standards. Paris and London, I think, were about that size prior to the plague and reached that size again perhaps early in the Renaissance. Some of the cities under the Umayyads and Abbasids exceeded 500,000, but they had a significantly different culture, with a strong middle class that was missing in Christian nations through most of the Middle Ages.

[Francis Buck]

300,000 would be an enormous city by Middle Ages European standards. Paris and London, I think, were about that size prior to the plague and reached that size again perhaps early in the Renaissance. Some of the cities under the Umayyads and Abbasids exceeded 500,000, but they had a significantly different culture, with a strong middle class that was missing in Christian nations through most of the Middle Ages.

You're absolutely correct. I've researched a little more, and have done some refining (with a special thanks to waldronate for the Medieval Demographics Made Easy link -- I had that page in my bookmarks quite a while ago before losing it, never was able to remember the name). I'm currently putting the entire "nation" of Orthigar at two-million, with the capital city of Sivan holding about 75,000 people, and a standing army of 40,000.

Predictably, another question has come to mind: what's logical for garrisons? If Sivan, walled-city, has 75,000 people, and Orthigar in total has about 40,000 standing soldiers, what would be a logical number to divert into the defense of said city?

[Gumboot]

The classic Medieval Demographics Made Easy (a good read, by the way) states that a typical settled region would support about 180 people per square mile. That population density would work out to 7.2 million people in the 40000 square miles of the region if everywhere was settled.

I think that's probably a bit high. I wouldn't take population density (over large areas) any higher than 120/sq mi.

300,000 would be an enormous city by Middle Ages European standards. Paris and London, I think, were about that size prior to the plague and reached that size again perhaps early in the Renaissance.

300,000 is ridiculously big. London was actually pretty average for a medieval city - it had less than 50,000 people at the height of the Middle Ages and didn't top 100,000 until the 16th Century. Paris was much bigger, hitting 250,000 in 1328, but that was the largest city of one of the largest and most densely populated countries in Europe, at the height of a population explosion.

You're absolutely correct. I've researched a little more, and have done some refining (with a special thanks to waldronate for the Medieval Demographics Made Easy link -- I had that page in my bookmarks quite a while ago before losing it, never was able to remember the name). I'm currently putting the entire "nation" of Orthigar at two-million, with the capital city of Sivan holding about 75,000 people, and a standing army of 40,000.

That's very unrealistic to me. There's actually an excel spreadsheet which has the data from that article, and you can plug in your kingdom information to determine population, city sizes, and so on. I use it for my world building, and I have a kingdom of 9 million with very fertile land (i.e. very high population density) and the largest city is only 45,000 or so. They also have a standing army, and it took quite a bit of fudging and manipulation to make 18,000 a believable number. 40,000 would be an extraordinary large military even for a modern country of 2 million. Not that it's impossible; it's a smaller percentage than the Roman Empire, but you have to allow for some considerations:
1. The Romans had such a large army because it was a way for immigrants into the Empire and newly conquered peoples to gain citizenship
2. The Romans had such a large army because they were an incredibly aggressive conquest-focused state
3. The Roman model was ultimately unsustainable; they couldn't support their armies long term and this ultimately contributed to the collapse of the Empire

Predictably, another question has come to mind: what's logical for garrisons? If Sivan, walled-city, has 75,000 people, and Orthigar in total has about 40,000 standing soldiers, what would be a logical number to divert into the defense of said city?

A feudal kingdom with a standing army would almost certainly have restrictions on where that army could be deployed (i.e. only on the frontier) because citizens and nobility tend to get nervous when the king/emperor/sultan has an army sitting around getting bored. The defense of the city would most likely fall to a special unit, for example Roman Legions were prohibited from entering Rome (actually they were prohibited from entering mainland Italy at all) and protection of the city was the responsibility of the Urban Cohorts (who were the police force) and the Praetorian Guard (who were the Emperor's bodyguard). That's why Rome got sacked so frequently; if an enemy force actually managed to make it past the frontier of the Empire, there was nothing between them and the capital.