I made the same move but at 75kg and went with a 100L custom Rider quad (not intended for freestyle) which has seen me out in 10 knots catching fun lumps at the local like yesterday with a 6.1(Dutch Inn). I can uphaul this when the wind drops so no worries in light wind and pump on to a soft wave. Now that my skills have improved I will eventually upgrade to a smaller board around the low 90L for light winds. I had a more skilled , lighter sailor out with me yesterday that was on a 5.7 / 85L combo and he was going just as well (or better ). So I would suggest a low to mid 90s volume (25 L over bodyweight) but if you are serious about getting in to freestyle you may want to get a Freestyle wave board rather than a dedicated wave board. Then as you progress you will probably go smaller.
The big advantage that quads have at Dutchies is smaller fins to avoid the rocks (although I still found a few the last couple of days on a low tide )

My recommendation would be to go for a thruster rather then a quad. A 90 liter board should be a good start. You don't want to go to big. The bigger boards tend to have much tricker rails. You'll be struggling pushing the rails in with 65 kgs. Just my 5 cents... Better still see if you can test some boards and see what works for you before you commit hard cash.