This information describes farming methods using a three-field system and a heavy plow. So if a family needed 35 bushels for the year and got at least two bushels per acre, the minimum farm size to support that family would have to be 17 acres of arable land, plus pasturage for oxen and other livestock and the peasants' cottage(s).
A family of four needed about 35 bushels of grain a year. Of the seven to ten bushels that a peasant harvested on each acre of his land in a good year, two or three had to be saved for the next years seed, three or four had to be given to the lord as taxes and to the church as a tithe. In the end, a peasant would have between two and four bushels per acre to feed his family with. If the harvest was not good, he would have even less, and his family might face hunger or starvation during the winter months.
Those numbers roughly line up, particularly as the average family size was probably larger than four.
Yardland - A peasant holding in the common fields, usually 25-30 acres of arable land with appurtenant meadow, pasture and common rights.