The way that I approached this was to create a hierarchy of towns - from hovel to megalopolis in 16 levels - and decide on a population for each type of town. Then I put in a bunch of different size towns on my map and added up the numbers. If the numbers seemed too large then I cut the numbers down per town and re-added things. That drove me crazy after a while as it was totally obsessive and anal. So now I just use about 3 or 4 sizes of towns and give them big numbers and just say that the rest of the population lives in small unimportant villages. This way I can say that the country has a few million people without getting so obsessive about placing all those towns and villages and hovels and cities and etc.
Then I base my military upon those numbers, roughly 25% of the population can be called into service (that's a number I decided on so it may not be accurate). So with 2 million people that would give you an army of 500,000...quite large but is a max number of total able-bodied men no matter their age. For a standing army I use a number like 2.5% which is 50,000 and reasonable since those guys are spread over a vast countryside...some patrolling, some in forts, some in cities, etc. with even the smallest village getting at least a squadron of soldiers (roughly 5 to 10 men).
If you want lots of monsters roaming the countryside, a la Points of Light, then you just make up whatever number you want...in this instance take it down to 1 million people total with about 200,000 in big cities and towns with a standing army of 25,000. This leaves lots of open land for monsters to roam around in which makes the countryside very dark and dangerous. The reality of the medieval period was far more prosaic since there were no monsters (just bears and wolves and large cats) the population was much much larger, hence the seemingly skewed numbers. The reality in which fantasy is based on is very boring so you have to cut down the numbers of people to make it work.