I know that every kingdom is constructed out of hexes and it's the basis for how large a kingdom is. This goes into what improvements it can have, how many, where the settlements are, ect. But one thing I'm having a hard time finding is how large, physically, those hexes are. Even if I can get a ballpark number, I need something to try and outline the scale of the land mass that I'm trying to create and how many hexes to break it up into. A good example of known landscapes and land masses with hex grids on already would also be helpful.
Like the exploration system, the kingdom-building rules measure terrain in hexes. Each hex is 12 miles from corner to corner, representing an area of just less than 95 square miles. The hex measurement is an abstraction; the hexes are easy to quantify and allow the GM to categorize a large area as one terrain type without having to worry about precise borders of forests and other terrain features.
In every other game I've ever played, hexes are measured by their edge to edge distance, because that is the way you move from one to another. For whatever reason, PF says their 12 mile hexes are measured corner to corner, which gives an edge to edge distance of about 10.4 miles.
The area of said hex is a bit over 93.5 square miles, which is close to 100 square miles, a nice round number. I suspect that is the reason why. The area of a hex 12 miles across is a bit under 125 square miles.
In my works, I have used the edge to edge distance (like everyone else in the entire world does) of 12 miles. This is exactly 4 hours to walk across for someone whose speed is 30, or allows 2 hexes of travel per day along roads without having to force-march. (For people whose speed is 20, this is 6 hours to cross a hex, and for those with a speed of 40 it's 3 hours to cross; very easy math for the bulk of situations.) It's also a lot closer to 125 square miles than a 10.4 mile hex is to 100, meaning I have vastly smaller rounding errors.