My relatives who run a farm (some tens of cows, pigs, chickens, various crops for sale and food) do it on I believe around 25 acres of farmland plus a few acres of forest (for firewood). They are able to sustain a family of 5 people (3 grown ups, 2 school age kids) this way, plus hire a few seasonal/part time workers.

It does require quite a bit of machinery though- several tractors plus all the attachments (plowing, harvesting, cutting etc).