You can also ask @[email protected] , he wrote about medieval demographics years ago
DIYRPG Worldbuilding
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.
Also remember that good farmland usually was hard to find, so the actual area is much larger. Depending on where and when, the farmland would be like 20 - 80 % of the actual area, with the mean closer to the bottom than the top. And it would be arranged in a circle around the town (who host the farmers market). This will obviously depend on the terrain ofc, but a rule of thumb is that a farmer should be able to travel to the market, be there for a while and then travel back home the same day. Beyond that limit there will be fewer farms, and if well populated, there will form new market towns withing their travel distance.
C&S has had charts based on medieval sources for working out the sizes of demesnes since at least 2nd edition. The current edition has very detailed lists toward the end of the book.
Have you considered more than farm plots in such domain? I think a forest, patch of a barren land or a quarry could make it two to three hexes.
I always think in such case about a polish book in which a group of villagers fought against lome local knight/lord over the ownership of a local forest.
Sure, but that's a case by case situation that doesn't affect the total land area use.