Driddle said:
Quick, without researching the information (and without actually posting the answers here, because we don't really care either)...
* How often to you actually think about the vast number of people directly near you (or not) as you drive to work each morning to earn your weekly owlbear-slayer salary?
* Do those numbers make your life any more or less "real" to you?
I'm going to cherry pick these two questions. I think about the vast numbers of people in the area I live in every time I get into a massive traffic jam at 12pm on a SATURDAY afternoon (which is every Saturday). It is immediately apparent just how many people there are.
As for the second part, I don't really think about it. But if I were to be playing in a game set in a roughly modern world, with 8-lane superhighways leading into the city, and my DM told me that only 270,000 people lived in a 100-mile radius insted of the 2.7 million that there actually are, I'd be immediately forced to wonder how there could possibly be a traffic jam.
I fall in the logical, medieval level of population. It seems like a good default. If you plan on changing it to something else, you should have reason, however simple.
And to further answer Driddle's questions - of course I don't know off the top of my head. But I can find out the answer in five minutes. If I'm going to present something in my game, I try to do some basic research if seems like it will be an issue.
Population density has never come up for my players, that I know of. It has occurred to me many times, as my early campaigns were vastly underpopulated. I just started thinking, "you know, after several years of this, my PC's have depopulated the kingdom of just about every logical evil NPC of their level I can reasonably expect to exist given this level of population."
Then I looked for info, and found my assumptions about population were way off. I adjusted my campaign accordingly. My players maybe unaware, but I'm happier.