Anyone have any advice on how to make nations, their borders, etc?

Following Rivers and Mountains are the obvious thing to do when deciding borders, but that is tedious and how do you know when you should follow a river and when not to.

I also can't find any data for Kingdom sizes of the middle ages, but my search-fu sucks to begin with. Right now I'm ending up with states with an average size that is about 240,000 km^2 which seems too big, but I'm up to 130 and I've only done 3/4 of 1 continent. This seems a bit much considering there are only around 250 states on Earth presently...

So any advice or tips?