Suggestions on a width for a keyhole cockpit for the average person?

Last night after we got in a couple of the guys paddled my new boat. It has a 16W x 30L cockpit. To my surprise one of the guys that is about my size, just a bit stouter built, had to slide into the cockpit. His hips were touching the sides of the 16" opening and he is not a large man either. I never considered myself small either.

Steve on the other hand is skinny and of course had no problem getting in with room to spare.

I realized that this was to small for a lot of bigger built people. So I need to make a form for a wider cockpit but I am not sure how wide to go. I like smaller cockpits but that obviously isn't going to work for a lot of people. So I am looking for some suggestions. I will probably measure some of the plastic boats next Thursday but I know that some of you guys have ideas.