Don't sweat it. It would be sacrilege to come to sandy Point and not partake of the speed bank.

Winds range the whole gamut from whatever low number you can sail in to the occasional Nuclear storm! 12m to 2.5m should cover it!

But you will have a lot of fun even if you smallest sail is a good 5m on all but the biggest days. If it was stronger than that you would probably feel more comfortable on a smaller board anyhow.
Winter is not the best time for sailing here. The days are short so the tides can be wrong all day. It is also pretty cold, especially the wind chill factor on a good day. Also, winter is not a great time for wind. Mostly it is very unfavorable Nor' Westerlies which are a real pain at best.
I would recommend delaying you trip until the Sandy Point Speed Fortnight event from Sat 26th Sept - Sun 11th Oct. This is when the wind patterns change to more favorable conditions (More Sou' Westerlies) and the days are longer. And plenty of guys around to chinwag and sail with.