And as you surely now, Gary, a section is what you could get for free from the US government for settling the American West, after the Homestead Act of 1862.
I believe the requirements to stake your claim were building a 10' x 12' dwelling and living there 5 years (or was it 4), before you got the title. I think you had to make your living off the land, but I'm not sure if that was a legal requirement or just what everyone did with 160 acres of Nebraska.
So I figure 160 acres is a full-sized American semi-arid farm, what a man could work (with some horses) and run a few cows on, with a "back 40" and so on.
And I figure 40 acres is a decent sized small farm in a developed area with more rain, from the short lived policy of giving freed slaves "40 acres and a mule" at about the same time. I believe that happened in the areas the Union Navy took in 1862 -- the Sea Islands off Georgia and South Carolina, and maybe parts of Louisiana?
But I wonder how many acres a typical medieval peasant farmed? I'm guessing there's no real answer to that, as it would vary with geography (a lot smaller plots with productive land in the Netherlands or the Thames Valley, a lot bigger with rocky soil in Norway or dryer land in Poland).