Hi, Dimitri and Craig,
I'm Closer weight (around 70kgs) to you Dimitri and as Craig said it depends of what you want to do. For example I use my blast 115 in 13/15knt to 18nds with 6,7 on flat water and with the same wind in wave conditions, my wave board with 5,4 (freswave stb 85 or Quattro 99 if it's very light wind and wave)
I Hope it helps you.
Guillaume