It depends a lot on your climate and the length of your growing season, and also what techniques you use. Google "square foot gardening" for some good tips on how to maximize garden real estate.
Also, if you're inventive and enjoy an experiment, aquaponics (again, Google it) is a really awesome way to use fish to grow plants quickly and in a small space, with very low water use.
In a good growing area, with the right techniques, you might be able to supply a person or two on 0.1 acre (400 square meters). Less-ideal locations will not be as productive.