Marc,
Bring everything! I was there 2 weeks late Aug/early Sept 2001 for the Canadian Championships. Some days, a 9.6 was not enough for Formula racing.
Another day, a 4.2 on a wave board was fully lit up for slalom racing. Pleasure sailing, I would have been another sail smaller.
My best day there had to be the perfect 5.5 wavesailing day at Corfu beach - 8-footers, side-on conditions. Did my first back-loop (did not set out to do one, it just happened)
Stay anywhere on the lower part of the islands and you are not far away. The lagoon has good flat-water, protected sailing. If you are into waves, the wind direction (and strength, current, etc) will tell you where to sail.
Lagoon water is very warm, ocean water is rather cool. Air temp can be around 10-25C, 55-80f. I think it was generally 70s each day.
You may pick up some more info at the following site:
http://www.coupedesiles.com/
Have fun!