Unfortunately, most of the time people are writing about medieval social structure, they almost never include any numbers to give any kind of reference for the scale of things. But from pieces and fragments I've come across on various sites (without sources) I pieced together these general guidelines I want to apply creating domains for my setting.
A common borderland domain has 1 lord who lives in a small keep with his family and some 20 to 30 servants. Next to the keep is the town with some 1000 to 2000 townspeople, who are the merchants and craftspeople for the area.
Surrounding the town are 20 to 30 manors that each support 1 "knight" and around 5 fully trained and equipped men at arms and their families. The lands of the manor are being worked by roughly 200 to 300 peasants who might be free farmers paying rent for their plots, or serfs and slaves working directly for the "knight"'s own household.
That would mean such a domain has 1 lord, 20-30 "knights", 100-150 men at arms, 1,000-2,000 townspeople, and 5,000-10,000 farmers. (Allowing for a peasant levy of some 1,000 men if the domain is attacked.)
At 3 acres of farmland to support one person, this would be a total requirement of 20,000 to 40,000 acres or 80 to 160 km². Which is one or two 6-mile hexes.
Does that seem broadly plausible as a general reference for scale?
Perhaps this calculator can help you: donjon; Medieval Demographics Calculator
It does produce similar ballpark numbers for the total population and largest town given the same area of farmland. Which is nice.