Originally Posted by
gioscinelli
Also, shorter length more spin, longer length more power to the pedals. Pros usually go with a little longer length for both spin and power cycling.
There is a more recent favor of longer cranks than what was convention 30-40 years ago for a given rider, in general. But there are many variables as already suggested.
For a fixed gear conversion, a road frame to fixed, I would go 165mm as the bottom bracket will be lower than from a track frame, say, 70 mm of drop being very typical.
For a fixed gear conversion, it will be all about chainline. Probably driven by the rear wheel.