Here is my standard cockpit. I am 6"-2" with a lot of legs and like 4" clear behind my back to the coaming so my cockpit is 33" long (and 16" wide).

You also see a woodworkers' trick for drawing curves (an equal number spaces on both sides with the spaces being equal length per side, on the two intersecting straignt lines tangent to the start and end of the curve; connect points between the spaces with straight lines).