What benefits/disadvantages come with running shorter, or longer cranksets?

The length of a crankset is more important on a fixed gear. The reason for this is you are less likely at striking the pedal on the ground when turning with short cranks than longer cranks.

Most fixed gear riders use a 165mm crank, I personally use a 170. I'm 6"2 and 165's feel tiny to me. I kept this in mind when picking a bike and got one with a bb height designed for fixed gear riding on the street.

Another reason many fixed gear riders choose a shorter crank is that it makes it slightly easier to spin at faster candences. This helps particularly when going downhill.

With a longer crank you don't have to apply as much force as with a 165 with the same gearing. You do however push the pedals farther per revolution.

If I ever build a road bike I'll go up to 172.5 or 175 cranks.

I used to run 165's, now I am using 172.5's. I can't really tell the difference.

https://www.myra-simon.com/bike/cranks.html