First, capacity. You need at absolute minimum 30L. Xieheng goes with 35L, smallest in the team. I went with 32L before, which is fine for day-climbs, but becomes uncomfortable trying to haul for multi-day trips. And you end up with way too many things strapped on outside... A 40-50L that can compress down (to 30-ish L) is a great, versatile choice. Look for compression straps on the side, which can also be used to strap on snow picket/tent/thermarest/ trekking poles/etc. Seco