The guy is wanting to sail in 20 knots. That is pretty windy. I'm 95 kg and would be using a 7.5 or 6.8 in those wind conditions. A 100 litre board would be fine. I can uphaul my sonic 95 with no problem.
If you want to to just blast around and have some fun a sonic 100 would do you fine. They are very cheap and are as fast as newer slalom boards if not faster. A 32 techtonics falcon goes with it very well.