I read during this break a book called The Measure of Civilization by Ian Morris, which is basically an analysis of the figures behind his other book (which I didn't read) Why the West Rules - for Now.
There are lots of different and interesting figures but I would like to point one that might interest all of us (either as RPG players or BT ones).
Pre-estate agrarian societies seem to be able to support settlements of no more than 10,000 people.
For agrarian states, it is 100 000 people ; for agrarian empires, 1,000,000 and the industrial societies are for now (since the growh didn't end yet) at 25,000;000 people.