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I have one of pic related. It's 3L, and sometimes I hook up a hydration hose. Additionally, I carry a 1L bottle.
If it's a short hike, I can use the extra weight anyhow for practice, even if there's no remote chance it'll all get used. If it's long, I don't regret having it. If it's a group hike, I'll often add at least another liter to this. It seems there's usually someone along who underestimates their usage.
Overnight generally means some kind of purification. Steri-pen is my favorite for ease of use, but I carry something for backup. The steri-pen plus iodine is easily light enough for even day hiking.
