If you're thinking of adding a porch to your home, says H. Dale Conart (Remodel), you need to consider the style of the house and how you plan to use the space; and he recommends that a front porch be a minimum of 8 feet deep and 16 to 20 feet wide, while screened back porches have grown from 200 square feet to 300-400 square feet.
I just dug out my house plan to compare. Am I on the cutting edge?! Okaay. My front porch is 44 feet wide (the width of the house), and it's 8 feet deep, though it widens to 12 feet at the doorway, which is recessed. (I love that recessed space! It has room enough for an Adirondack chair on each side of the door.) My front porch is good. (Sigh of relief. You can get a real inferiority complex from home shows and magazines.)
My back porch is also 44 feet wide and 8 feet deep, but the recessed area is only 2 feet deep. (I can still fit an Andirondack chair on each side of the French doors.) Oh, oh. If I've done my math right, that's 264 square feet. Oh, well. It's still a fantastic porch, even if doesn't have ceiling fans... (When discussing porch ceiling fans with our builder, we decided to save money by not installing them. Also, we wouldn't have to worry about them in hurricanes.)