We usually go in July-August. I'm not sure if the wind is different in May. I mostly use a 6m in the morning and 7-8.5m in the afternoon sailing at Malcesine/Navene. If you sail Torbole in the afternoon I understand 6-7m is more normal.
At Malcesine you'll have a light wind 'wobble' out to the wind line in the morning so you may need a slightly bigger board - I'm heavy (95kg) and usually sail a 110 litre board in the morning - others use much smaller. 130+ litres is ideal for the afternoon at Malcesine.
If a storm comes through (and they often do) you'll need smaller usually 4.5-5m at Malcesine.
Have a great week!