The size of the backpack mostly depends on its purpose and the duration of the trips that you are going to take. So let’s discuss different types of packs and their usual sizes so you can know what to choose.
Daypacks: Most of these packs range from 20 liters to 35 liters on the high-end. For half of the day, you will most likely need a smaller bag. While for the whole-day trip, you may need a larger, 35L sized backpack. However, for winter one-day trips, it is much better to use backpacks with a storage capacity of 40-45L.
Weekend backpacks: For overnight trips, it is more convenient to take some larger packs than daypacks as you will need a lot more gear. The backpacks that will be perfect for that purpose should have 40-50L of volume.
Multi-day backpacks: The main difference between the weekend option is that you will need to carry much more food supplies for more days. It means that a 50-70L backpack will be just fine.