NPR has a story on custom-gardening--outfits who come in and set up your garden for you. They claim a 150-square foot garden will keep a small family in vegetables for a year. I suppose it all depends on your definition of "keep". If you eat at home every other day, maybe. If you're eating lots of vegetables every day, I doubt it. Our garden is nominally 500 sq feet, though probably closer to 400, once you allow for the compost bin and path. It keeps us in salads from late April to November or December, but we still buy vegetables (like potatoes and jicama) during that period, and tomatoes up to July.
It's easy to over-sell this stuff.