Take any standard manufacturer's single seat length & add 50%!
I have a supposed double jockey. I reckon in the real world it's more like a 1.5man jockey. The SR5.4 "three man" I still reckon takes 2 comfortably......
(one of) This wintler's project(s) was to extend the base it sits on, move the cusion back about 6", cut it in half, move the rear portion back a further few inches & fit twin backrests, the front one having a handhold, thus loosing a load of useable deck space, but at the same time creatuing a handy out the way place for the dry bag.
Option 2 was to just make it a 1 man, which is a whole other debate...