Hi Foxi, hmmm good question, was just at Garda for 4 days now, blasting on a 5.0 and should have been on 4 or even 3.5....so good Ora at Garda is easily 4.0, otherwise 5 or even 6. And Walchensee a proper thermic is good with 5.0/5.5, a poor/light one approx 6?
Choosing a wing quiver is easier from the other end, ie your smallest, so if you do not need smaller than a 3.5, then a 3.5/4.5/5.5 make a good range for most people and conditions and would fit the 6.5 size well too. 2 Wings total is always possible, but you would need to give me more info if this is your plan or if you have other sizes, skill level, foil size, weight etc?