a pack in the 50-60 liter range would suit you well for many things you're looking at (2-3 days) and as you progress and pick up lighter more compressible gear it'll get you to 4+ days and supplement with a smaller pack for shorter pursuits.
you'll find many suggestions and none of them wrong. good brands to pursue (short list) Osprey, Wild Things, Cilo Gear, Arc'Teryx, Black Diamond, Mammut and more.
look for a pack with not too many bells and whistles, top loading, ice axe loops, good hip belt and shoulder straps, and fits well.
i like Osprey packs mostly due to their fit for my torso as well as Arc'Teryx but dont like the price tag.
something you could transition from backing to modest mountaineering will give you a good bang for your buck. Osprey Aether series, Black Diamond Quantum, Gregory Z series come to mind.