There are a couple of resources. There might be some data in Sigurd's analysis of Herefordshire: http://www.cartographersguild.com/sh...light=hereford
Also, the article Medieval Demographics Made Easy is a good source, and there are some associated calculators linked in at the bottom of the article.
The rule of thumb I use for medieval populations is that an acre of decent farmland can support 4 people, and 5 acres of pasturage can support 2 large animals. That's for typical medieval grain farming techniques. Different kinds of subsistence can be better or worse.