Just in front of Westerland you have the Brandenburger Strand (I guess its name was like this), which is the surfer's beach.
You an go out there but be aware that the police might get you off the water, because it is not allowed to fly any kind of kites there.
Then just go a little bit up north or south, then it is no prob.
Winds: south (side) west (on) north (side)
Sizes: depending on the wind, which can be light ot storm (so bring all of your stuff)
or have a look at the forecast.
Best spot for me is the "Ellenbogen" all the way to the north, you need a car.
Shallow (but lots of muscles, so wear shoes!)
best for east, south, west winds.