I lived in Phuket for a few years. Phuket in August can be good for kitesurfing. Usually there is west wind. One of the best beaches in August is Nay Yang beach, since there the waves are not as aggressive because of the reef protecting the beach. Several kite schools are there. Kite Zone/Kitesurfthailand is a good one. Bring your bigger kites.
What is the weather? It is the rainy season. My biggest kite is 15, and 12 and 10