I can answer all your questions, Ash.

Lundland is approximately 200,000 sq. miles, not acres. In the last game, fief size on the map was entirely arbitrary, due to the map needing to be made very quickly after our map guy flaked out. This time, I plan on having each's player's fief roughly correspond with something logical. 

Using medieval technology and agricultural methods, a square mile of land could expect to support 180 people, if the land was fertile. Lundland isn't very fertile, and more comparable with England, so a single square mile will likely support 100 people. Infertile land will likely only support 50 people per sq. mile, and scrubland 30 at the most. This in mind, each Fair fief will likely be 250 sq. miles, an Infertile fief 140, and scrubland 100. It's no perfect measurement, but it'll do for our purposes. If the map doesn't fill out completely, fief sizes will be changed to fit the mould.

As for crop production, you're right, and that's the point. People aren't supposed to have numerous large towns. The largest town a single Fair fief could support on its own would be 1000. But we're assuming here that you have multiple fiefs with which to help feed your non-town population and soldiers, which is why it is sometimes necessary to forgo more elite fiefs in favour of more numerous, lesser quality fiefs. 

If you wanted to sustain a town of 6000, then you'd need a total population of 40,000 people living in Fair fiefs.

Also as an aside, 1 sq. mile is about 640 acres, if'n I'm not mistaken.

EDIT: Ashgan's right, it takes 5 bullis to buy the thingy, and 1 bulli in upkeep to maintain it for every season the levy is raised.

And yes ulster, just add up your wealth as best you can. Unless you elect to play a mercenary group, there's no way you can use points to directly contribute to your military.