If surf a lot at lake garda in Italy. I use NP Combats, i.e. wave sails, for freestyle (4.2 - 5.3). Many italians who are really light use a North ICE 5.0 or 4.7 for freestyle - even some guys who obviously compete in contests. I would say that up to 5.3 you can easily use a wave sails for freestyle,too. If you go bigger, I would suggest to take a pure freestyle sail.
Also, you may want to use the smaller sails in real wave conditions and with a waveboard. And this way around a wave sail is far better than a freestyle sail...