I've been there already 2 times and will definitely go there again with my sun this year at the end of August. It has become one of my favourite spots.
My weight is 86kg and I use 80% of the time sails between 6-7 m2.
During the months of July and August, the thermal wind called Meltemi, is almost every day present and blows the strongest in the afternoon.
There are days that it really blows 25+, but it's for sure not the average. The average is more like 15-20 knots.
I always rent my equipment there, so I can choose between different boards and sails.
If I had to choose 1 board, I would definitely take the 105 liter board.
In your case, with your weight, I would choose the 124 liter board.
Keep in mind that when the wind picks up it gets quite bump & jumpy. Not really waves, but sometimes nasty chop, high enough to launch you into the air if you hit it steep and hard enough.
At Blue Horizon (the best hotel & location for windsurfing) the wind blows the strongest and perfectly side shore. The F2 Pro surf center is right at the beach of the hotel.
For the wind, there's only 1 better place on the island, i.e. Prasonisi, which is completely in the south. There you have at least 10 knots more of wind, but unfortunately no decent hotels. It's a desert, good for wild campers.
Hope this helps.