I ride 175mm cranks on my fixed gear mountain bike and 170mm cranks on my fixed gear street bike.

With an appropriately high bottom bracket, as one finds on dedicated fixed gear bikes, pedal strike does not pose a problem.

With a converted bike, though, 170mm cranks can hit, with disastrous results.

You can ride with 170mm cranks on a conversion if you remember you have cornering limits.

As for the effect of the length on spin, my 170mm cranks spin so much better than my 175mm, but my 175mm cranks give me easy torque for very slow speeds on uneven ground.

For a street bike, the shorter the cranks the better.

The rider's leg length has almost nothing to do with crank length.

Rather, as one goes shorter in length, one trades off torque for spin; and, as one goes longer in length, one trades off spin for torque.

Shorter cranks spin much faster than one would expect from 5mm increments.

I'd rather have spin than torque, and so I gear down a little and spin much faster.

I have stayed with 170mm cranks on my street bike only because I had my bike professionally fitted with 170mm cranks; otherwise, I'd go with 165mm.

I ride with 175mm cranks on my mountain bike only because no one makes short cranks for mountain bike bottom brackets.