Mike Burrows and Mark Stonich --both of whom have an interest in recumbent bikes-- are both proponents of shorter cranks, both for recumbent riders and for the rest of us.
On my recumbents, having smaller cranks do seem to relieve stress on my knees far more than longer cranks do. I was running 155's, now 165s, and I might go back.
But on my road bikes, my 170's seems to be OK. I can usually find a gear combination to allow me to do 95-100 rpms. I might stick to this size simply because most of the brevet routes around SE MI tend to be rolling terrain, and I could use the mechanical advantage going up hills (especially at my weight).