Good advice from Alex and Mark, and unfortunately, this one you will have to figure out for yourself through trial and error.
We all "scramble" when arriving at the beach and are trying to figure out what sail to rig. All of the below come into play including asking all your buddies what they are using and on what board.
Skill level of sailor
Fitness level of the sailor
Board (weight, width, type)
Type of sailing (speed, slalom, longboard, formula, bump and jump, freestyle, wave)
Wind speed and direction
Sunny or cloudy (clouds tend to kill the wind)
Water state (smooth, chop, waves, boat traffic)
Racing or freeriding
Limited time at the beach or the whole day (rig big if you only have a short time)
Goals (fun, top speed, racing, jibing/tacking practice, etc)
Point of sail (reaching or upwind and downwind)
Type of sail (RAF or cambered race sail)
Water and air temperature (do you want to fall in or not?)
Choosing a sail size requires skill and experience to get it right, and even the most experienced get it wrong from time to time.
Make notes every time you come in from a sail. Eventually, you will build your own chart of what size to use.
It's all part of the windsurfing experience. Have fun.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.