If I start world building, I often don't know when to call it good and just start the damn campaign already. Luckily, S. John Ross has an excellent article on medieval demographics, and there is also a calculator based on the article to do the math for you. Just plug in your town names and you have a good chunk of the work done for you. I've used this to stat up the two kingdoms of Avengard and Oerlund for my campaign world.
Truth be told, I love online map and world generators. A page called Irony Games once hosted my favorite map generator, but it has since been swallowed into the Stygian depths of the Internet and I have yet to find a suitable (and, more importantly, free) replacement.