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.