I use my 6.5 up to around 20 knots but as a beginner I would probably limit that to around the 15 knot mark until you get a little more experience. If you are game you can alway try pulling on a lot more downhaul and trying it up to 20.
If you are going to get a smaller sail, go for maybe 4.8 - 5m for those days of 20knots +.
Welcome to your new addiction