WHAT SIZE?
A 15-20L pack is usually fine for most day walks, because it can carry a waterproof, a warm a warm layer, some lunch/snacks, a first aid kit, a hat and gloves, a map and a compass. 25-40L packs are good for
2 or 3-day walks Some larger packs have a zip which you
can open like a suitcase for easy access. where you want to pack some overnight stuff (and are also good for family walks). 45-60L packs are for trekking, when you need absolutely everything for a multi-day journey, possibly including camping kit.