Crank Length Test
When choosing a crank length the first concern should not be any theoretical advantages of a long lever arm vs a high cadence. Much more critical is our hip flexion.
Test: Put your bike on a trainer. (Assuming your saddle height is correct) Ride in the drops on your road bike or in aero position on your TT bike to warm up. Stop pedalling and pedal backwards veeery veery slowly. How does it feel when you get your foot over the top of the stroke? Do you sense a “locking” in the hip.
If getting over the top is just as easy as the rest of the stroke then fine… If it is hard, one reason may(!) be too long crank arms. Try with your hands on the hoods, still there?
The key is to back pedal very very slowly. Feeling slightly or heavily stuck on top of the stroke means you are probably pushing knees out in every normal pedal revolution and placing additional pressure on your lower back (not to mention the hips themselves). What is left is for the weakest of the links to reveal itself!