First Time DM Question: Main City/Town

[Nalkubar]

So, I'm new to DMing. I'm running a campaign for two friends that are also new to the game, so we've been having some fun getting used to the rules and generally blundering around. Some of the stuff that I've done in my noobishness is mistakenly lead them to believe that there is a conspiracy in the town guard (which there isn't), and accidentally give them a poison strong enough to drop an elephant. Now that we're into the third session, they're nearly done with their beginning quest. I'm going to have to start fleshing out the world, and the real test of my DM mettle is about to begin. The problem is that so far, I've been pretty vague about the size of the town they're in, and the capital city that their adventure is based around.

I've done some research about what the population of a capital city should be in medieval times, and I've got everything from 50,000 to 750,000. My question is this: how large should the main city of a kingdom be? Also, what kind of settlements would realistically be around a capital? For reference, the area that the kingdom occupies is around 100,000 sq miles, and the city is the center of regional trade. I'd appreciate any input.

[Altair_the_Vexed]

Coincidentally, I'm working on a blog regarding settlements in fantasy, right now!

It will depend on the population you think is right for your kingdom, which will depend on the geography you have in mind, and how much land is cultivated / used for herding / etc.

But for a quick decision, you can just look at historical examples. This article on Wikipedia (http://en.wikipedia.org/wiki/Historical_urban_community_sizes)gives a list of historical populations of various settlements.
You can use that for a guide - it'll help to see what sort of city had what sort population too. For example: in the late 1500s, the two largest cities in the world, Beijing and Istanbul, both major centres of trade and political influence, had massive 700 000-ish populations, while the next largest cities were around 200 000.

So - pick how big your city is in the world scale, select an era your game-world is equivalent to, and then take a number off the chart!

[Thrawn4]

Right. First thing, medieval times are a very broad term, and medieval facts are hard to come by. My first advice would be: Don't do too much research. After all, it's a game and meant to be fun. Also, it's fantasy, so you are allowed artistic license. The size of a city is difficult to estimate, so you can wing it by "it is said". My suggestion: If it is a small country in a low fantasy setting, make the city smaller, and vice versa in a major country in a high fantasy setting. But it is your game, so do what you think is best :-)

[nedz]

The size of your city can be whatever you want: as big as your piece of paper is a reasonable guide. Having some vague notion about the economy, trade, food production etc. helps but you're only rally interested in verisimilitude; i.e. does is seem believable ? Any more work than this is going to be wasted and since only about half the prep you will ever do will be ever seen (if you are lucky) the less you do the better.

You should focus instead on creating a conspiracy in the town guard. Player led plot is a thing, and it's a plot line they have already bought into.

[JusticeZero]

mistakenly lead them to believe that there is a conspiracy in the town guard (which there isn't)Maybe there should be, then. It's an easy enough adventure to create, and clearly they have enough clues to start unearthing it.
accidentally give them a poison strong enough to drop an elephant.Woohoo. They can kill one thing. Maybe whatever it is that is behind the conspiracy is hard to kill, but there's a fairly obvious way to slip it some poison.
the area that the kingdom occupies is around 100,000 sq miles
An area that large is going to require another layer of management to actually run places, and that's the layer that you will actually be interacting with most of the time. As far as the size of the city, that will depend on its ability to leverage military and economic power on a large scale. The ones on the high end of your range there were cities that had some oceanic imperialism going on.