What I've noticed is its a comfort issue. My standard go to crank is a 170 anything longer tends to slow down my spin. Shorter than 170 feels like walking around with the shoe laces of my right and left shoes tied to each other.

Most persons do not experience that when, for example, climbing stairs. For example, 175 mm cranks from top to bottom of stroke is just under 14 inches but a typical stair height is about half that:6 - 7.5 inches. When going to shorter cranks some may find that shifting to a higher gear feels more natural: like climbing stairs where they are 'lifting more weight' but doing it is smaller steps.

Only on a fixie (or did you mean break?)...

My attempted humor.

Back to the OP, I have 172.5 on 1 bike, until the left crank arm broke. I had a spare 170mm laying around, so I put that on temporarily ....... 3 years ago ..... Can't tell the difference.

I'm 5' 11" tall l and have a very long thigh for my height and a quite stubby torso. I ride 172.5s 175s and an old tourer with 177.5 cranks. The 177.5s would normally be pretty long for my height but they seem to work OK with my thigh length. I like the longer crank on distance rides. I ride at a lower cadence and cut down the reps for less chafing. Some folks probably do the opposite, but that does seem to work for me. I ride all three though on and off with all being fairly comfortable.