I have used the Square Foot gardening technique on and off for about 10 years. This year I wanted to do trials of soil remineralsisation treatments (see the 2008 trial pages for more information.) I used 1m square beds and sub-divided each bed into 4 500mmsquares. I grew a separate crop in each square.
Onions seemed to do particularly well with the 4 squares producing an overall yield of 36 onions weighing just over 10kg. This shows that a significant amount of food really can be grown in a small space.