In Colorado I met a guy touring through from the west coast who thought 190 mm pedals were a good idea. We swapped bikes the day he stayed in Denver and we rode up to Boulder. I felt a sharp difference and geared down a little to ease knee strain. After ten miles he couldn't stand the way my 165s felt and we swapped back. People with really long or short legs might need longer or shorter cranks so their legs and joints would have an optimum range of motion. Maybe?