"one writer in a major magazine article quite a few years ago claimed that after considerable testing with many different riders, 18.5% of the distance from the top of the femur to the floor in bare feet should be the crank length." http://www.peterwhitecycles.com/fitting.htm
I just measured myself... and apparently I need a 185mm crank... (I'm 6'4").
I think that would pose some problems with pedal strikes... but I haven't gotten my frame yet.
Do most tall people just run 170mm cranks? seeing as a 185mm could be hard to come by.