The size that you choose for your screened porch is going to depend on your own personal needs and what you think works the best for your home. Screened porches can vary in size so you'll need to use your judgement. Most homeowners want a screened porch that is as big as possible without taking away from the yard. Some of the most common sizes include 18 X 14 and 20 X 20, though there are a lot of variations depending on what will work for your home. You may want to invite a few contractors out to look at the home and help you decide on the right size for your screened porch. Your budget may also impact the size screened porch you can afford to build. Gathering estimates can help you determine how to make the most of your budget and build an appropriate sized screened porch. This can also give you an idea about how much you will need in financing before you get far into the process.