Shorter is better is the general rule. I haven’t done that experiment for over 40 years since my system has dictated the length since then. A small (but important) improvement as I remember.
However, my most trusted audiophile friend and source says there is a magical really short cable... in the 18" (monoblocks positioned right next to the speakers) where speakers really come alive due to the short connection. I had monoblocks on loan for a couple years, but never wanted to buy expensive speaker cables to find out how great a difference that was. However, if I had monoblocks... I would do it and find out.