I've heard that the general rule for fins is 5x sail size in m2 + 3. So according to this for your 6.5 something around 36cm would be right. However, for a tail that is so wide I think this would be too small (especially with the straps in the outboard position).
According to these guys you're going to need something around 38cm.
Hope this helps.
JP Super-X 106l, North F8 Ram: 5.8m,