: Since you're building it, you can make a cockpit whatever size you'd like!

But make sure you can get a spray skirt to fit. If you just make a comfortable size opening, you may not be able to find a spray skirt to fit it - or you will end up paying a lot more to have one custom made. Some of the recreational plastic kayaks have "large" cockpit openings - and the maufacturers offers a spray skirt. So find a size you like and copy it.