Just rode faster than the previous 4 rides with my new 177.5mm dura ace cranks.
The crank length was recommended to me by the fit calculator on competitive cyclist.
Having recently tried 165mm, 172.5mm, and 177.5mm, I believe your crank length should be set by both your physical size, and equally as important, your riding style.
Masher = long crank length
Spinner = short crank length
For Mashers, longer crank=at a given RPM, you can push about one harder gear for a given amount of power.
For Spinners, shorter crank=at a given RPM, your actual footspeed is slower (Smaller circle) for a given amount of power, thus saving energy.