I've never heard of a 171.5 crank; 172.5 is pretty common, though. And I agree, that's a little on the long side. But I can't imagine you'll feel much difference with a 170, the difference is really negligible. Unfortunately hardly anyone makes cranks shorter than 165, and even those are hard to find.

I use 140 mm BMX cranks. For other options, take a look at this.