In order to decide the size of your greenhouse, you'll want to take stock of you're available space, and how big the plants you're planning to grow are, not to mention how many you're going to grow. The square footage can be determined by how many plants you will need for your household. A lot of the time, simple research can give you all the answers you need to get started.
As a rule of thumb, a greenhouse of around 12 feet by 24 feet can successfully feed the average family. However, if you were planning to include fruits as well, you need to consider making the greenhouse larger to fit fruit trees. Fruit trees tend to take up more space than smaller vegetables.
