Backpack capacity is measured in liters, ranging from 12L day packs to 80L+ packs for winter camping and mountaineering. A 55L pack is a very common size that may work for some backpackers but not for others.
55L is plenty of space for lightweight backpackers and summer trips. In warm climates, you don't need to bring as much gear, so you won't need 70 or 80 liters of space. A 55L pack will work great for most people going on a weekend trip in spring, summer, or fall. A 55L pack will also work fine for a lightweight backpacker that has their kit moderately dialed in.
A 55L backpack is big enough for most backpacking trips for most people. If you are backpacking in the winter, or planning on carrying over 7 days worth of food, you may consider something a little larger. , The capacity of a backpack needed for backpacking can vary a lot depending on the length of the trip and how much gear is being carried. A great "do it all" capacity for a backpacking pack is between 50L and 55L. This is enough capacity for longer trips or a lot of gear, but isn't to large for shorter weekend trips as well.
A 55 liter pack is suitable for almost any backpacking trip regardless of the length of the trip or how much gear is being carried. , Explaining backpack volumes: For a day hike a 20-30 Liter backpack is large enough for a change of clothes, lunch & snacks, and a water reservoir without being too big or heavy. Since overnight gear is not needed, there's really no reason to go larger. If you are skipping the clothes, then 10-20 L is enough.
For backpacking and overnight hiking, the size depends on how bulky and heavy your gear is. For 3 season hiking, 50-60 L is large enough for a weekend hike or a thru hike. If a bigger pack is used, it will likely be filled with stuff that won't be used. "Just in case" gets heavy fast. A lighter backpack makes for a more enjoyable hike.
For winter hiking, expeditioning, etc. 80 L should cover it. Winter clothing is heavier and bulkier. Sleeping bags are bigger. A +100L backpack should be setting off alarms on whether the gear is needed or not.