Ocean Blue- Where do you live?
I would use a weedfin of at least 40 cm depth; possible as large as 50 cm depth. Very wide-tailed boards like the Go require long fins to trim correctly, even with smallish sails like a 5.8. It has to do with leverage. If you got a too-short weedfin, like 36 cm, it will be hard to stand in the footstraps without the board tilting sideways and your heels dragging in the water.