It's amazing how gardening works. It's basically area times water times sunshine. Only stressful moments are cold nights and spontaneous storms.
I wonder how many resources an average consumer needs to get enough harvest for a full year of sustainable consumption? A lot of land and water seems to be necessary.
I have a friend with a couple of basic hoop greenhouses (around 8x2m) and for 6 weeks in summer they have more than enough tomatoes and cucumbers to provide for themselves to eat, to preserve and to give away to friends.
I think with a little planning, and only eating whatever is in season that week, you could easily grow enough in a 100m2 garden to provide all the vegetable needs for a family during summer. Planning is the important bit though, as some crops like broccoli need to be planted the year before. Oh and it's gonna be a hell of a lot tastier than what you get in the supermarket.
Our greenhouse is 60m2 + vegetable garden around 200m2 + few dozen apple trees and some other fruit trees. The season is not very long here (NE Europe), so we basically start to harvest lettuce, radish and spring onions some time end of May and from that moment on we basically switch to "home grown" diet if you wish which lasts into late October or so.
The part about tomatoes and cucumbers is very true. Once they start to bear fruit, you need to have your preservation or distribution plan ready or they will be wasted.
That said, we have to visit supermarket weekly anyway...