Put a yard stick on the floor and sit on it. You should be able to see how wide your cockpit should be. The length required depends on the length of your legs, how much difference there is between the height of the front and back of the cockpit, and how easily you want it to be able to get in and out.

: How do you determine the size of cockpit needed for a Guillemot or Redfish
: kayaks. I am 5'8" and weigh (gulp) 220 lbs. I'm not sure from the
: beam dimensions if my 36" waist will fit in. Thanks, Jeff