Took a spin on a small Pivot full suspension mountain bike during the Tour De Steamboat last weekend. The FenderBag fit nicely on this 29″ wheel size small pivot frame; however, I do run some seat post height. If you like you dropper post to drop flush with the seat tube, the fit is tough.
I enjoyed the bike but I think I would prefer a 27.5 wheel size. Too much rake in the front fork or balloon-ish tire or something. The small FenderBag fits just fine on this bike.