Hey Harrow. Gotta agree with Pugwash here. Im around the 90 kg mark and I use a 6.2 wave sail as my biggest, then down to a 5.4 and 5.0. In fact I probably use the 6.2 the msot in waves unless it really honks.
Above 6.2 I go from 6.6, 7.5 and 8.5 race sails on a dedicated slalom board.
I often use a wave sail on a free ride board like at Botany in a good strong NE, lighter and easy to throw around.
At your weight a 5.8 would be fine for lighter winds and then down from there. If your going to try Gerroa next season get a 4.5