A good rule of thumb is to choose a bladder that will hold at least 2 liters (64 oz) of water. This will ensure that you have enough water for even the longest hikes.
Another thing to consider is the type of terrain you’ll be hiking in. If you’re planning on doing any off-trail hiking, then a larger capacity bladder can be helpful, as you may not have access to clean water sources as often.
Finally, think about how long you’ll be out on your hike. If you know you’ll be gone for several hours, then it’s important to have enough water to last you the entire time. Bear in mind that you’ll also need water for cooking and hygiene, so always err on the side of caution when choosing a bladder size.