The single most common backpack mistake is buying by height. Two people the same height can need different pack sizes, because what matters is the length of your spine, not your legs. A tall person with a short torso and long legs takes a smaller pack than their height suggests.
Get this wrong and the pack rides on your shoulders instead of your hips. On Zion's long descents — and they are long, with 1,500 feet of switchbacks on some routes — that turns into shoulder pain, a sore neck, and a hike you cut short.
You need a second person and a soft tape measure.
That number, in inches, is what pack sizing actually uses. Most manufacturers group it roughly as small under 18", medium 18–20", large over 20", though the exact bands vary by brand.
A properly fitted pack carries most of its weight on your hips, not your shoulders. The belt should sit on the iliac crest, not below it. If you're loading the shoulder straps to keep the pack up, either the torso length is wrong or the belt is too low.
Day hikes in the canyon rarely need more than 18–30 litres — you're carrying water, layers and food, and you can't resupply, but you're back by evening. The Narrows is the exception: you want something you don't mind soaking and that keeps your center of gravity low, since you'll be balancing on invisible rock.
Overnight trips into the backcountry push you into 40–65 litres depending on season and whether you're carrying a bear canister and cold-weather insulation.
An empty pack tells you almost nothing. Load it with roughly the weight you'll actually carry — water is the easy stand-in — before deciding whether it fits. Most outfitters will let you do this in store, and it's worth the trip.