Fin for iSonic 111

Hallo! I'm interested in buying new fin. Which model is the best for me and how long should it be? My board is Starboard iSonic 2009 111 l, my sails: Gaastra Vapor 7,6 and Gaastra Vapor 8,6. My weight is 65 kg. This set is more for flat water or small waves. I want to achive maximum speed, I'm taking part in competitions.