Warlock,
In the Birthright rulebook, pages 33 & 34 has population info.
Basically, provinces are 1,000 to 1,500 square miles, 30 to 40 miles
square.
Level 0, wild, unsettled, less than 1,000 people
Level 1 to 3, thinly settled, main towns under 1,000, total 2,000 to
10,000
Level 4 to 6 10,000 to 40,000 people towns up to 5,000
Level7+, may get over 100,000
A bit more is in the last two pages (95 and 96) on domain design.
These populations approach historic medieval European densities much
closer than most … most FRP settings are grossly underpopulated
relative to earthly societies at that technology level.
Lyndon