Most standard touring sea kayaks have a depth of around 12 inches at the front of the cockpit and around 8 inches at the back. I used these numbers on the Point Bennett, and it's a nice crossing size cockpit with enough room to move around a bit during long hours on the water. Sleek Greenlander types, depending on how much of a cheater roller you want, will go an inch or two lower.

I'm 5', 10" and 165 pounds and like a cockpit 30" long and 16" wide, with 4" between the backband and back of cockpit to help with layback rolls. I can get in the cockpit butt first and then bring my feet in, which is great for cowboy scramble rescues and straddling launches and landings.