This is the first, and most important step to choosing your outdoor dining set. You'll need to be super exact with measurements to avoid any nasty surprises after you purchase the set.
Most importantly, consider what you will be using the dining set for. If you have a small family and probably won't be inviting guests over, you'll only need a small set. The more people that will use the set, the larger it should be.
Measure out exactly how much space you have to work with. When doing this, take into account at least 3 feet of space on all sides of the table to ensure people will be able to move around the furniture comfortably.
If you have anything that dictates the boundaries of your dining area, like railings or steps, stick to a smaller setup, even if you think you could squeeze in a bigger one.