At 5’11″ and 58 yrs I’ve found 160mm a big help: the 175mm crank on my ’99 Rans Rocket was too much on my knees and ankles. Got a new Rans Fusion Crank-Forward and opted for a Lightning 160mm crank; the reduction in range my knees have to traverse is really helping.
Fast rpms is something I trained myself for in the 90′s, and it came right back to me when I started riding again this summer. Shorter crank and higher rpms is a natural combo; I find I get a more even cardio workout and less need to over-exert on the hills. Still, I’m finding I do want to gear down more with a shorter crank. The work per revolution is 9% greater dropping to a 160mm crank; at 23.5 gear inches I drop to 50 rpm and more work than I want on the steep spots. I’ll drop my low-end to 17.5 gear-inches, which should let me get back to 70rpm, hitting the same speed with a lot less “work per revolution” on those steeps.