Backpack sizing

27L- 34L for day trips. Anything bigger might be bulky/better for multi day trips. I have a five year old osprey 32L pack that's my daily, shes very durable

I rock a 24L Dakine Heli Pro or a 25L Millet Big Powder. I haven’t found myself needing more space but it’ll depend on how much shit you wanna carry with you. I can fit my DSLR, a Nalgene, a few snacks, extra goggle lens, a few tools, and my avy gear without any issues. I have a helmet net and keep my jacket strapped to the outside of my pack as well if I get too hot, rather than trying to fit it inside the pack. I would say anywhere from 25-30 is a good starting point, but again you’ll want to evaluate what you really need vs what you can forego. Some people don’t mind extra weight for snacks food etc some people want just the minimal requirements. Take a bit to think about what you’d wanna bring with you and go from there. Hope that helps

I got a 16L Osprey basically for free for my first day-trip tour a couple years back and haven't bothered to upgrade yet. Fits my avy gear, puffy, small first aid kit, water, multi-tool, warmer gloves for downhill, gogs, sunnies, some GUs and a couple bars/snacks and small sammie/wrap. And a summit beer.
Would be pretty hard to fit anything more in there (shell has to get strapped to outside), but it forces me to pack minimal and it's nice and slim.

If I were buying new I'd probably enjoy another 10L or so.

24-30 is the sweet spot imo, I have a heli pro 20l and use daymakers and it’s a tight fit with the adapters.