A greenhouse can be made self sufficient at almost any size, as long as there is enough room for the equipment necessary. For a standard family, between 100 and 400 square feet of space should be plenty of room to grow enough plants to feed everyone through the year. However, the more space your greenhouse has, the more air there is to warm and more distance there is for air to cycle. Lean on the side of modesty when choosing the size of your greenhouse, you can always build another.
Exactly how large your greenhouse will be will depend on what your growing. Fruit trees and climbing plants will require more vertical space, while squash and root vegetables will need more room at their base.
