As you can see, it encompasses a pretty fucking huge area. 281 6-mile hexes are considered to be Civilized, while 332 6-mile hexes are Borderlands. That means that 10,624 sq. miles are considered to be Borderlands and 8,992 sq. mile Civilized. The book will explore further implications of Civilized v. Borderlands v. Wilderness from here on out, but for right now what we learn is that Explorers can only found their domains in Borderlands or Wilderness, while demi-humans (elves and dwarves) must found their domains either in the Wilderness or Civilized/Borderlands areas of domains founded by their own race.
Wales is 8,016 mi², for comparison.
During the Middle Ages, the largest town in Wales was Cardiff, with a population of 1,500 to 2,000 inhabitants - 300 to 400 families in ACKS terms, making it a Large Village (Class V). Cardiff would not be big enough to evenmake Wales "civilized", in other words.
Even as late as the 14th century, there were only six towns large enough to qualify as ACKS Large Towns in the whole of the British Isles - London (4,500 families), York (1500 families), Bristol (1,200 families), Coventry (960 families), Norwhich (800 families), and Lincoln (700 families). There's 94,525 square miles in the United Kingdom. If we assume 8,992 square miles per town, with no overlap, that's 53,952 civilized square miles, still leaving half the islands uncivilized. But of course most of these towns are actually clustered near each other in an arc across the east and south of England, with Cornwall, Wales, Scotland, and Ireland virtually absent of urbanization. In ACKS terms, England would be largely "Civilized", while Cornwall, Scotland, Wales, Ireland, and the smaller isles would be Borderlands and Wilderness. The rules tend to create urban clusters with borderlands and then wilderness and assume adventure is happening at the borderlands. This is in contrast to the "points of light" in contemporary D&D, where cities seem to dot the landscape with huge wilderness in between.
Since Every Campaign is a Law Unto Itself, you can, of course, assume a smaller radius of civilized and borderlands territory if desired. An approximate downward limit of size that an urban settlement can control is a number of hexes equal to (urban population / 31), based on a 10:1 rural:urban ratio and an average population density of 310 families per 6-mile hex (50 per square mile). London would need 145 6-mile hexes or 4,500 square miles just to feed itself and the peasants who feed it, excluding any smaller towns in its orbit. Anything less than that and you get into bad simulation - where does the food come from?