You will need a tape measure and pencil.
Stand inside your porch with your back to the house. Sketch a layout of your porch.
Start numbering your openings with the opening #1 at your left adjacent to the house. Continue numbering the openings clockwise.
Make certain you include a door as a number opening.
Upon sketching out your future porch on paper follow these same steps.
Measure in 3 locations horizontally and vertically in the locations as shown.
Take the smallest of the three measurements you took vertically and the smallest of the three measurements you took horizontally. These two measurements will be your opening size.