I was using the standard Naish fin (see photo). It is bigger than it looks, it has a lot of area and it is quite thick and very stiff. I have put my surf fin alongside for comparison. I thought this fin worked well for sailing, it was enough to get my 100kgs up wind, while staying loose which is what this board is all about. I am not sure about bigger sails, they may end up burying the front of the board and with no footstraps you don't want to get overpowered. I don't use my board for sailing any more, I covered up the mast track when I put my deck pad on and only use it for SUP now.
I like my 11'6" for surfing, athough now that I am improving, I am looking at changing to something which surfs better. It doesn't seem to like small mushy waves, but from waist high to overhead it works well.
Steve