Hey Erozion,
A 14m or preferably a 12m would do you fine for Perth Summer. I weigh 90 kilos and rarely use more than an 11m. If you go the 14m you'll definately need a 10m also as you'll get flogged on the windier days. If you only want one kite, I'd go the 12m.
Perth
regularly gets wind far exceeding 22 knots, especially in November/December and a 14m isn't fun in strong wind.
Good winds,