Crank arm length does not matter much except in a few circumstances:
Longer arms give more leverage, which is an advantage at low cadence situations, like when you have to muscle over rocks, through snow, sand, etc. Otherwise, not so much.
Shorter arms let you spin, which is an advantage if you're riding all day.
Do not switch to much larger crank arms than your knees are already accustomed to; the increased motion will not be good for your knees.
I'm 6' tall and ride mostly on 165 and 160 cranks; I have 170's (and even 171's
on one) on other bikes and that's okay. I'd go with shorter if it were easy; but there is only a limited selection of cranks shorter than 165 mm.