Cook also recommends hexes that are six miles across. This works pretty well as it has our heroes crossing three or four a day. There are some other issues to keep in mind as you’re picking your scale.
One is sight distance. You don’t want the PCs to be able to see all of a single hex, especially if you’re going to be using wandering monster rolls. You may want lairs, camps, or even just the critters themselves to not be seen the first few times the PCs move through a hex (which helps explain why they’re suddenly popping up now that the dice say they should). Six-mile hexes mean each hex covers a bit more than
medieval demographics calculators at the Domesday Book, a town of 5,000 people covers 83 acres, which is 0.13 square miles. Lots of room to lose small towns or villages (or orc camps) in a 6-mile hex. London in 1200 AD is assumed to have had a population of roughly 25,000. The Domesday Book page gives us a size of 412 acres, which is 0.64 square miles. At that same time, Rome was assumed to house 9,000,000 people. That may be too large for the Domesday Book page, and it returns a size of 148,258 acres or 231.65 square miles. That comes, very roughly, to 6-and-a-half of our 6-mile hexes. Paris’ population in 1200 was 110,000, which the Domesday Book page says should have covered 1,813 acres or 2.83 square miles, which fits comfortably in our 6-mile hex while still clearly dominating it.
The 6-mile hex works great for a muscle-powered world. If you want a world where people travel by jet-cycle, or live in massive cities like Tenochtitlan (possibly 212,500 people in 5.2 square miles not counting the greater metropolitan area) you might want to expand the size of you hexes. If, however, the world is full of dense jungles and tiny villages, a smaller hex (maybe 3 miles across, roughly a league) might be more appropriate.
UPDATE: More praise (and better math) for the six-mile hex at "The Hydra's Grotto."
Art by Ivan Constantinovich Aivazovsky and Alberto Pasini.