One set of estimates of populations throughout time:
http://geography.about.com/library/weekly/aa011201a.htm
Note that 1 million was reached before 1000, but by 1000 (AD/CE) the largest was 400k or so.
Doubt that you could truly cross Kaineng City in 15 minutes if you lived there, that would mean the entire map was maybe an hour across. Even at beyond marathon speeds we're looking at 10s of miles across the entire continent, a bit small.
Shameless Plug for another section of the forum:
http://www.guildwarsguru.com/forum/f...play.php?f=221
In particular, the Second Lore Project has a number of posts about size of continent (real time walking vs. time crossing if you lived there, population of Tyrian cities, etc.)
If we go by estimates from therein of thousands or tens of thousands in Ascalon City/etc., we might get a million in Kaineng City. Its a bit more spread out, but not 100 000 times larger in size, much less the tens of millions (assuming 10-100 times more people just due to higher use of a z-axis) times larger we would need for trillions.