12m about the average sized kite everyone uses up around goldcoast / bris area. wind is hit and miss during winter, and is often all or nothing, with the occasional gem of a day, but you got to be in right place at right time, and ready to drop everything at short notice. big westerlies are good, but probably suited better for windsurfing as they can be pretty full on with big gusts - 1.5 to 3 ml steamer best imo, but some folk do it without anything
cairns in winter is tops, warm solid wind from SE, and again i think about 9 - 12 m kites is typical? go in the nude
if all else fails, try the kite forum