I am 6'5 and use 170 or 175mm cranks.
don't remember my cycling inseam, 37" or something like that, maybe a bit more.
170mm cranks on all my c&v road bikes. 175mm cranks on my 2 modern road bikes.
I dont want anything longer...i haven't been riding and wished for longer crank arms. I haven't though 'oh man, some 185mm arms would make this fun!'