In my limited experience, you can't necesarily look at where your knees end up just in front of the opening. I am 6'2" 200 lbs and there are quite a few kayaks that I just cannot get into not because of my height, but because of the length of my legs. Also, since the front of a cockpit is tappered, your legs cannot both come up and out of the front few inches at the same time (or get down in). My guillemot opening measures 16 x 29, but in reallity it is only 27" long and is a very tight fit to slide down in with my legs pressing against the deck as my but slides over the coaming.

I have considered cutting the opening bigger just so I can bring my knees up to stretch like I can in my double. This is the hallmark for my next kayak.

Bottom line...make it narrow for a nice fit and long enough to bring your legs up when seated.

Tom