This will go into all the steps I use to get a country and what information you can get from the process
1: In autorealm or the program of your choice create some squares. Make them a pretty color and set your scale such that said squares are 100mix100mi
2: Place your squares. A small kingdom has 1-5, a large European state in the 1600s has about 10-40. An empire has as many squares as you think is reasonable. There are a number of ways to place your squares. Randomly by hand is easily enough, but you could also pick a section of real-world map and place them on that if you prefer. Don't worry about the details of each square. We'll go into that later. For now just place them. I've provided an example below. Pay no attention to the numbers.
3: Now, establish the population in each square unit. I rolled 1d6. On a 1-2 I assigned a population of 100k people, on a roll of 3-5 I assigned a population of 300k people, and on a 6 I assigned 500k people. Now that we've laid down population densities we might start thinking about features, etc. The block of low population density up top might be mountains or desert for example while we see that all 3 areas with a population density of 50/sq mi are on the coast. Perhaps they're merchant cities or naval powers. Certainly, they dominate the country. They're also where the largest cities will probably wind up. We might also divide the country up into administrative divisions (provinces, duchies, or sub-kingdoms) of about 1-6 units. I'll skip that step for now, but it's something to consider.
Things are taking shape now, so let's digress for a moment to talk about the characteristics of a 100x100 mile block (I'll call it a county) and it's subdivisions:
-It's about 4 day's walk end-to end. It could be less with horses and more with wagons or an army. If the area is particularly inhospitable, this distance might be larger. That's something you'll have to figure out when drawing your map.
-It has about 1 barony per 20k people with a barony being about one day's journey on foot from end-to-end. We'll talk about placing baronies later, but in principle it's okay to have empty space where there are no baronies. In areas with a particularly high population density baronies will be smaller in size.
-In turn, each barony has many villages with between 100 and 1000 residents. (Assume about 500. This is a bit high historically speaking but you can adjust it on the fly). Therefore each barony has about 40 knights and petty nobles who run manors and villages. So now we have a choice to make: how many villages are manors and how many have no lords. Arbitrarily I'll say it's a 50/50 split (I have no idea in RL)
-So here's the exiting part! Assume a county population of 300k. If so there will be about 15 baron-households and 300 knight-households. If we're talking a European-like country where being a knight is a big deal, this tells us a lot about the military capabilities of a county—how many knights it can raise (300 knights +100 non-heads of households who are knights, and +100 similarly equipped and mounted mercenaries), etc. In a D&D pattern world where realism is out the window it's unclear. I'll proceed using my own setting assumptions. Just be aware that they won't square with D&D's.
-These 500 people will make up the backbone of the county's army, supplemented by infantry and peasant conscripts. If we assume that most people are too malnourished or busy farming to pick up a PC class these knights will represent a large fraction of martial PC classes. My assumptions follow:
If 1 in 600 are knights, barons, or otherwise able to serve as mounted knight
So let's say that that accounts for about 1/3 of the county's PC fighters. The table I used based on that follows (if you use all of however many classes there are in 3x these numbers are useless. Assume these classes are all there are in the world):
Fighters: 1 per 200 people (1/3rd of these are knights)
Rogues: 1 per 200 people
Rangers: 1 per 400 people
Barbarians: 1 per 400 people
Wizards: 1 per 800 people
Clerics: 1 per 800 people (I assume that there are many non-spellcasting members of the clergy. If you don't, clerics should be more common)
Total PC classes: about 1 in 50 people have a PC class of some sort. That's about 10 per village of which 5 are level 1 characters. Works perfectly!
To calculate levels, assume that half of all characters of a particular sort are level 1. Half of the remaining fraction are level 2, half of that fraction are level 3 and so forth. For example:
750 L1 Fighters
375 L2 Fighters
187 L3 Fighters
93 L4 Fighters
45 L5 Fighters
23 L6 Fighters
11 L7 Fighters
6 L8 Fighters
3 L 9 Fighters
1 L10 Fighter
The Kingdom of Lestone looks something like this:
4000 Knights and about 2500 more people who can serve in that capacity
This squares nicely with historical European capabilities to field knights bearing in mind that not every single knight can be mobilized at any given time.
Lestone is a fairly large and populous country by medieval standards—in line with England and Wales' population in the 1500s though with significantly more land area (much of which isn't fully inhabited)
In the next section we'll talk about placing baronies, towns, cities, and castles within a county. I'll also throw out some suggestions for ways to semi-procedurally add geographical features.