You can make coamings any size you like. My cockpit openings are all 33"x19" keyholes so I can use the same skirt on all of my kayaks. The length should be long enough to allow you to sit first then pull your legs in (I am long legged). This makes the cowboy (scramble) re-entry much easier. Also it helps you to get out of your kayak quickly in the beach backwash before you get sucked backwards into the shore breaking waves and even facilitates doing a commando landing with your skirt off and feet out before hitting the beach. A wide opening, i.e. coaming near the width of the sheer of the kayak, makes a kayak easier to high brace, balance brace or scull. A low coaming also helps. Both help keep the kayak from rolling over on top of you when your back is flat on the water. Having 4" clear between the backband and the coaming also makes lying back easier. Be sure to locate the paddler in the kayak in relation to the center of buoyancy then locate the coaming off the paddler.