What determines crank length?
As an opener to your question....


Femur length. 
Theoretically. The longer the femur - the longer the crank length.
The old pros used to favour the longer crank lengths 175mm to 
180mm being common. Iirc Pantani who was a short rider even 
favoured a slightly longer 172.5mm for reasons of leverage as you 
mention above. There has been a trend towards shorter crank 
lengths - which lend themselves to a slightly faster cadence.
Natural cadence speed.
Most riders find when they switch to shorter cranks, their cadence 
increases. 
Flexibility.
A longer length will bring the upper leg at the top of the stroke closer to the abdominal cavity.