Well, it depends on the size of your backpack and your daily need of water. Some small daypacks can carry water bladders up to 2 liters. But other larger backpacks can easily carry liter water reservoirs. When it comes to water reservoirs, 1 liter, 2 liter, and 3 liters bladders are available. If you think that 1-liter bladder will suffice your thirst, you can go for that. But hikers and backpackers usually opt for 2 or 3-liter water reservoirs.