Medieval Demographics Made Easy
useful site I found today
Medieval Demographics Made Easy by S. John Ross
for working out just how big your populations would be - check out the calculator at the bottom of the page
I found that site by looking at this one from Geoff Eddy Creating an Earthlike Planet
which also covers help in creating cultural details. Check out the 'other pages' links at the bottom of the page; a bunch of useful stuff there.
- apologies if I'm repeating info already put up here somewhere.