Pack size

Heavy or light?
Big or small
85, 70, 65, or 40?

Trying to decide what pack size to get for myself. Hopefully I’ll get a look at an Osprey Aether 70 on Sunday. Thinking about a REI Mars 85 also. Got my son a REI Ridgeline 65 which should be plenty big for him and grow with him.

Any thoughts or recommendations