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?

Do you have some advice on how to evoke feeling of harshness in the setting without making it caricaturally grim or simply stingy with resources? Something like tolkienesque First Era Beleriand with inaccessible gods and great danger nearby but with less superpowered elves and less focus on nobility?

I've been thinking for a while how it could be very interesting to explore a fantasy world that does not take it cultural background from the Renaissance, High Middle Ages, or Antiquity, but instead imagines an environment similar to Europe in the 6th and 7th century. Between the collapse of a centralized government and Roman administration apparatus and the conquests of the Carolingian dynasty in France, Germany, and the Low Countries. A period that really isn't Antiquity anymore without the Roman Empire as the superpower in Europe, but also does not yet match the image that comes to mind when thinking of the Middle Ages.

What are some things about this period in Europe that you think could make interesting elements for the worldbuilding of a fantasy campaign?