If it were me- I wouldn't go over 19', I would use hips plus two fists, and I would use the balance measurement method from Cunningham to find the seat location (actually, the isserfik location), and then I'd follow Petersen to get the depth and rocker profile.

BUT--------- that depends on where your "big" is. Big guys can have their weight high up in the shoulders and chest, or they can have it around the middle, or they can have it down low. The higher your weight is carried, the deeper you would have to sit in the water to be stable, and that would take experimentation, as in multiple kayaks before you find out what paddles well. It's true- the "anthropomorphic" stuff only works for average weight distributions.

If you're adventurous and don't mind being a little different you could build an Alaskan SOF like a King Island kayak. They were originally intended for carrying large loads. A friend of mine built one and he couldn't paddle it without ballast. It wouldn't even sit flat in the water without weight in it. It would list to the side. Weighted down it would paddle like a dream. Check out Harvey Golden's Kayaks of Alaska.

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