I think as a rule of thumb, I'll use "a day's travel across" as the maximum for a single spot of terrain. We'll assume that an easy day's travel involves a horse, or roughly 20 miles, or given a perfect square 400 sq. miles, or 400,000 acres. Immense, yes, but reasonable. Sure, some places might be a bit bigger, some a bit smaller, but that seems reasonable enough to use as a rule of thumb. This puts it on the small side of an U.K. county, which I'm okay with as that allows us some room to play with additional terrains to make a bigger county.
So using that, let's pull some land types, stealing mostly from a Birthright adaptation for 3.5:
- Wasteland - Desolate, can't support any real existence. You might be able to find a few people who eke out an existence here, but you wouldn't want to be one of them.
- Desert/Tundra - Little rain, little to subsist on, no real ability to build cities, or even villages, without the addition of some feature that allows for it (River/Oasis/Lake/Coast).
- Mountains - The land of windswept peaks and cliffsides. Ore and stone are most prevalent here.
- Forests - Deep heavily wooded forests stretching for miles, and places the light never touches the ground.
- Hills - Little mountains, rolling hills, stone and ore can be found here, as well as the ability to raise some crops.
- Plains - Flat open land without anything to break the wind, perfect for raising crops, raising animals, building cities, and thus very much desired and fought over.
Right now I'm trying to avoid numbers till I figure out what I need to figure into my needs and wants.