Who are you going with? If it's Sunsail you can request a board and rig as part of the onboard kit.
Not familiar with the Saronic Islands. Would think the prevailing summer wind will be the Meltemi. Usually N or NE building up to a good f4, occasionally f5 by mid afternoon so unless there are some topographic features to accelerate the wind I think you would be better with a 6m.
Can you get in touch with the flotilla leader to ask about conditions? The holiday company must at least have a phone number for the flotilla base.