I've been suspicious of those crank shorteners for exactly the reason Tourist in MSN mentions. Perhaps ok for experimenting, but I have a wide Q-factor on my cargo bike, semi-FAT, and dislike riding it.

Origin8 has several crank lengths for a reasonable price.
https://www.amazon.com/Origin8-Alloy.../dp/B00R6R3ZPM
https://www.amazon.com/Origin8-Tripl.../dp/B00R6R5PBE

Usually it is best to match the right and left cranks, but it is also possible to mix lengths in special cases.