You're right, it's hard to put a figure on how much land you need. Before we bought our place I wanted 40 acres at least, as that's what I thought we would need. We ended up buying 25 acres of not great land, but OK, with an awesome water supply. We've been running close to 100 chickens and half a dozen sheep for a year, have an orchard with about 50 trees, an area set aside for a vege garden, and have seriously barely used maybe 6 of those acres, including the acre or so that holds the water. I reckon we could leave the back 15 acres and never go near it.