A medium sized garden can easily support the average family. Researching what dimensions are needed to grow your crops is recommended. Along with thinking about dimensions, considering what kind of crops you want are also important. For example, certain fruit trees will need a lot more room than spinach crops.
You can absolutely grow enough to feed your family. Seasonal crops can be supplying you year round. You can alsp harvest crops and store them efficiently to feed your household through the changing seasons. Making sure you have a cool place, such as a cold cellar, so store cans and containers would be the most effective way to provide food year round.