I think the ballpark you are looking for would be Biosphere 2 [0]. This is a 3 acre enclosed system that supports the food and oxygen needs of 8 people, althoug

I think the ballpark you are looking for would be Biosphere 2 [0]. This is a 3 acre enclosed system that supports the food and oxygen needs of 8 people, although the people lost a lot of weight and oxygen levels dropped dangerously. While this (somewhat failed) experiment is in the ballpark of what long-term spacecraft will need to sustain generations of crew, it also shows how much land is needed to supply the air we breathe. Notes: 0 - https://en.wikipedia.org/wiki/Biosphere_2 https://en.wikipedia.org/wiki/Biosphere_2