According to at least one estimate, the population of Japan during the height of the sengoku period was around 18 million. This figure was drawn, for the most part, by evaluating rice production on a province by province basis (a bit like what Candidgamera suggested). This figure gradually increased to somewhat less then 30 million by 1700. The population remained more or less static from there until the Meiji era. Between 1868 and 1912, the population is thought to have jumped from 30 million to 50 million. Needless to say, 50 million was reached long after the events depicted in Shogun, though 'Rodriguez' could be forgiven for his hyperbole - at least one 16th Century Western visitor to Japan described the population as 'innumerable' - as well it must have seemed to a European.
Looking province by province at koku production can give at least a vauge sense of the population of each. According to a nifty map provided with the Oda Nobunaga volume of the Rekishi Gunzo series, for example, mountainous Hida Province yielded some 38,000 koku as of 1598. Omi, on the other hand, was listed at having the potential to produce 775,379 koku. Tiny Shima province produced 17,854. I've seen a similiar map that lists the number of soldiers raised in 1600 on a province by province basis - if I can remember where I saw it, that might also be useful for making some general assumptions about the military capacity of each province.


[This message has been edited by FwSeal (edited 06-03-2001).]