> How much space does the average person have in their back yard?

Not nearly enough. In Sili Valley, I live on a 8000 sq ft lot. In much of the USA, people live on 1/4 to 1/2 acre lots. A garden that will produce enough fresh produce during the growing season to feed a family of 4 during the season along with enough to preserve for the off season is bigger than that lot, even before you subtract space for the house. I invite you to do the math.

My mother's garden fed us the year around. That garden was bigger than my current city lot.