From this web page: http://www.mnsu.edu/emuseum/history/...ages/farm.html

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.
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).

According this site: http://home.olemiss.edu/~tjray/medieval/feudal.htm :
Yardland - A peasant holding in the common fields, usually 25-30 acres of arable land with appurtenant meadow, pasture and common rights.
Those numbers roughly line up, particularly as the average family size was probably larger than four.

The Romans did not know crop rotation or the heavy plow. Those things came into use over the course of the five centuries after the collapse of the Empire (~AD 400. Alaric sacked Rome in AD 410). They were both in use throughout northern Europe by the time of Charlemagne, though, in AD 800. The heavy plow was largely responsible for the northerly shift of power in the Middle Ages. Prior to its introduction, much of that land was not arable, limiting population growth.

Hope that helps!