@quixoticgeek @thebiologistisn
7.2 sq meters with a cold frame is just about enough to make sure you've got a veg to go with most dinner for one or two in a mild climate.
250sqm should get you about a ton of potatoes, which comes out to about 2000 calories per day, or a quarter of what you need for a family of four. It should be more than enough to cover all the veg needs for a family of four, but you still will need carbs and protein.
We've got 5 acres (~2 ha) in central Arkansas that we're slowly converting into a specialty produce farm. I ran calculations once, speculating about sustainability. We could, with luck, support around 6 people, with enough spare to sell.
To do that, we'd still need to bring in about 1 ton of nitrogen, and about half a ton of other nutrients. We'd also be basically doing nothing but tending crops all year.
I've seen claims in books that you can fully feed a person on 300-400 square meters. I've never heard from a person who has actually lived that claim.