In my experience a fly with stiff wings is more of a "twister" than any variant. When I fished the #12 GFVs or dun variants, I typically used 4X. With 14s and smaller 5X and occasionally 6X was not a problem. The big skaters (like the one shown, which is on a short shank 16) are another problem. Fishing these doesn't involve as much casting as it does letting them ride the wind and skitter over the surface.

EP