evila said.. I'll be there in October/November, so hopefully it will be blowing by then.
I never kited there and I'm not taking gear (hope to rent).
Some good info here
kiteboardingflorida.com/ Checkout Miami Beach, Crandon Park in Key Biscayne, and Islamorada & Curry Hammocks State Park in the Keys.
To rent you need an active IKO certification at virtually all stores, its pretty pricey though. You might be better off buying a cheap board, having it shipped to your lodging, and then selling it on the way out. If you are going to buy any gear this year, the US is the cheapest place to buy it as the industry out of Cape Hatteras will usually ship you anything free, gets here in 2 days, and they are aggressive relative to Aussie prices (I get stunned looking at Aussie price lists for the same gear). Take it back as personal so you can avoid evil VAT type taxes.
12m in Sept...only if you weigh like 50 kg. We get alot of light wind here and Sept is the tail end of our summer (no wind) season. The only times we get wind are tropical storms/hurricanes and thats also not 12m territory. More important than going larger on the kite though is going larger on the board.
October is a toss up, Nov is usually great wind wise, 20 knots days are frequent.
I currently live in Miami altho I am hoping to move down to the Keys for the winter :) Anne's Beach in Islamorada is my current favorite place. Strictly speaking Key Biscayne/Crandon Park is the most flawless beach on earth for kitesurfing but the heavy regs and the people kill it for me.