Was reading an article from Scott Saifer from http://www.wenzelcoaching.com/ in related to ways to finding the correct crank arm length for a particular rider. Nice read.
Testing For Crank Length (done on a stationary trainer)
Step 1. Lowest Saddle Position (LSP) for Smooth Pedaling Over the Top: Pedal with one foot while the sole of the other foot rests on the trainer for support. If you can pedal smoothly over the top of the stroke 20 cycles with no “clunk”, try the other foot. If you can do it with both feet, drop the saddle 3 mm and try again. If you can’t, raise the saddle 3 mm. Keep going until you’ve found the lowest saddle setting that lets you pedal smoothly over the top with each foot. Measure the saddle height from seat collar to saddle rails.
Step 2. Highest Saddle Position (HSP) for Smooth Pedaling Through the Bottom of Stroke. This part requires extreme sensitivity or a skilled observer. Pedal with both feet. If you are currently pedaling smoothly through the bottom of the stroke with no sense of reaching, rocking or stretching and with pressure all the way to the bottom, raise the saddle 3 mm. If you are already reaching, rocking, stretching or losing power at the bottom of the stroke, lower the saddle 3 mm. Continue until you find the highest saddle position that lets you pedal smoothly through the bottom of the pedal stroke with both legs. Measure the saddle height the same way as before.
Step 3. Making use of the measurements:
The Possibilities a) If HSP=LSP or HSP>LSP by less than 5 mm, your current cranks are okay but you cannot use longer cranks efficiently. Set the saddle height to HSP. b) If HSP<LSP, your current cranks are too long. To maximize efficiency, reduce crank length by 1/2 of (LSP-HSP) and round DOWN to the next lower size if the result is not an available size. Set saddle height to HSP with the new cranks. c) If HSP>LSP by more than 5mm, your current cranks are shorter than ideal.
Increase crank length by 1/2 of (HSP-LSP-5mm) and round DOWN to the next available size if the result is not an available size. Set saddle height to HSP with the new cranks.