As to the effectiveness of small gardens: When I was a kid, I had a great aunt and great uncle (in their mid 60s) who lived in town in Carbondale, Illinois, only a few blocks from a major university. They lived in one of those old, large, wooden, victorian type houses that you see in older towns, probably built around 1900.
The front yard was pretty small. The back yard was larger, but not real big. I would estimate that it was about 80 feet wide by 100 feet deep.
Anyway, they always had the ENTIRE back yard planted in a vegetable garden. Rows of beans, greens,black-eyed peas, tomatoes, potatoes, etc. And they worked in that garden every morning and evening, year round, except when there was snow on the ground.
They were originally brought up on farms, and they knew what they were doing. It seems like every other day, they were picking and bringing vegetables into the house. They canned several times a year. Their basement was full of shelves filled with the vegetables they canned.
It appeared to me that with this small garden, they were pretty darned close to raising all of the food they needed.
In the fall, they would drive out to one of the nearby orchards and buy a few bushel baskets of cast-off apples, peaches and pears (damaged, or too small etc for shipping), and then they would cut them up into slices and can those as well.
The only food they ever bought at the grocery stores were milk, eggs, butter, meat and cheese. I'm betting that if they lived in a more rural area, and had just two more acres of land, they could raised all of that as well.
Just some memories.
Mannyrock