What size hydration bladder should I buy: 1.5L, 2L, or 3L?

Specialist communities like r/hiking generally recommend buying the 3L and underfilling it when you don't need full capacity, since you can always carry less but can't carry more than the bladder holds. 1.5L is better suited to trail running or short day hikes where weight and pack profile matter more than range, while 2L is a common middle ground for half-day hikes in moderate weather.