REI sells the 3L bag and 3L hydration reservoirs. I don't like anything over 2L. They are too big and when full take up too much space in one area, making it hard to organize the pack. Plus I don't like one container for all my water, in case it breaks/leaks. I would much rather carry a single 2L and one or two 1L bags. I have two day packs, and use a 3L hydration reservoir in them, because I am carrying very little, aside from the water. The hydration sleeves work well for this. But on longer trips, the hydration systems are just too difficult to manage with all my other gear, and smaller containers work better.