For me, crankarm length has to do with the angle of my knee at the top of the pedal stroke. Too far flexed and I get kneecap tracking problems. If I'm going to be able to reach the pedal at the bottom of the stroke, then I need 165s. Your fitter should be able to measure your knee angle at the top of your pedal stroke and give you an idea of whether it might cause trouble, and if so, whether it can be remedied by setback or a different STA, or whether you need shorter cranks. That's how I'd discuss it with the fitter if it was me, in any case.

My 50 cm bike came with 170s and I swapped the cranks. I have short femurs.