Whats the minimum daypack size for a late July summit? 15L, 20L, 25L? I've got a 25L now which weighs 2-lbs 7-oz. I'm evaluating some options to get under 2-lbs but they're 15-18L.

Intending to pack 3L water, waterproof shell, down jacket, high-cal food, electrolyte packs, light medical kit, extra wool sock, poles and the little things like extra battery, lip balm, sunscreen, wagbag etc.