It has been mentioned how small of a difference it is between some of the crank arm sizes. One should note that as it relates to one's knee angle at the top of the stroke, this difference is doubled.

You adjust your seat to the proper height in relationship to your maximum leg extension on the downstroke. When using a 175 crank, your seat height will be set 5mm lower than if you use a 170 crank, because the pedal is 5mm lower at max extension.

Thus on the upstroke, a minimum extension, the pedal on the 175 crank will be 10mm closer to your seat, the 5mm from it being lower plus the 5mm from the crank being longer on the upstroke.

This is all fine and dandy if one's legs are long enough to accommodate a 175mm crank (or 180 or 185). However if they aren't then this is more than enough difference to negatively affect the knee angle and mechanical efficiency.

In the case where someone is better fit to a 165, then using a 175 puts the top of the stroke 20mm out of optimal position, that's 4/5 of an inch. We all know that adjusting pretty much anything on your bike, be it the seat height, the stem, the height of your handlebars, etc., by 20mm makes a noticeable difference.

In the case of my recumbent, going from the stock 170 to 155 cranks, changed this parameter by 30mm, or 1.2".