The horizontal saddle position determines the stroke force distribution within a cycle. A longer crank often means pushing the saddle forward, reducing the effective length. While a larger rider, in particular one with longer femurs, will envitably be better suited to a longer crank, an increase in crank length on road bikes also increases the total movement of the knee to chest on a rider with an absolutely smaller relative frame. On the other hand, to decrease the range of femur motion one wants to raise the saddle. One is constrained here by geometry.
Empirical testing is complicated by the time needed to adjust to a crank length.
The body will require some time to adapt to the change in muscle use, and the brain will need adjust to a new set of experiences and controls for the determination and selection of cadence. This hystersis apriori precludes any possibility for a simple laboratory test. The main reason that the 170mm crank length lasted so long was not due to lack of experimentation but because it does seem to provide a reasonable length. While longer femurs will be better suited to longer cranks these same individuals tend to also have longer feet, also contributing, with the typical raise heel style, to a longer effective crank length. Despite the above recomendations, many small cyclists have had great success with cranks larger than the sub-170 mm.