I am not speaking from much experience with doubles as far as paddling/tripping surf launching but have been around the retail/ rental/ teaching end of sea kayaks enough to toss my two cents in the ring, besides your working on my next dream boat.

I have a preference for long keyhole shaped cockpits that you can get out of quickly or stretch. So the option of having snug thigh braces and easy entry/exit is more a matter of shape than small or big. Seems to me it would be good to have coamings the same size as any other single that you use a skirt on. Also a standard sized center cockpit would enable you to use a standard sized cockpit cover.

I don't think you need to give up roominess or boat control with different sizes of cockpit coamings. Just keep a set of velcroed minicell hip pads (or those store bought ones in a strapped cordura pocket) for different size paddlers or different degrees of connectedness. Preventing ones flopping side to side in a double is probably the first area where one could gain control.

With regards to a particular size correlating to sealing well, my guess that has more to do with the type of skirt being used, whitewater boats have big coamings but snug bracing. A big wave can pop a loose nylon skirt off a smallish coaming easier than a tight neoprene skirt would come off a big coaming.

Just my two cents but it bothered the heck out of me that there was all this rental stuff,different pfds,paddles,etc. but to have a double that required a different skirt than 95% of most of the rentals just seemed dumb.

With that insightful comment it would be neat to have a much smaller center cockpit with a custom skirt made by Snapdragon! Just make the front and back cockpit coamings like any single that you like.