Hi Steely,
I'd recommend a 9m for your weight in Perth. It's a windy season this year. Either nothing or 25+ knots.

It's nice to have the nothing from time to time to give the body a nice rest.

Size of kite greatly depends on the brand and model of kite you are choosing, some kites are more powerful than others. We try and match the rider to the kite and that means we need to know how you ride currently, how you want to ride in the future, what board you plan to ride with the kite and style of riding, and also any other significant kiting moments that may effect your decision ie accidents/scares and injuries etc.
I reckon there's more to buying the right kite than just the size.

Cheers,
DM