First, a martingale equates to a dolphin striker, not a gull striker. Not the same.
To size it, you’ll need to calculate loads that will take knowing your sail area,
boat weight, etc Or you can spec per similar sized/weight other makes of catamarans.
BTW I think highly of textile
rigging but I’m not sure the gull striker is a good place for it, given the high, constant loads. Gull striker is one instance that would invite actual creep (as opposed to constructional stretch).