My experience with Achilles is that its not so much about total length, more about flow.
If you can provide the achilles with ample swimming current to keep it entertained and constantly swimming than it will be good. This isn't to say that people should go get a 60 cube and get achilles.. but that people who understand the fish can provide it what it needs.
I kept the same achilles in multiple tanks within a 2 year span. A 40 breeder for 2 months, 180 for 8 months, 55 gallon for 3 months and back into 180 gallons for the remainder.
My experience was that his colors were some of the most vibrant, he was the most active and fat in the 55 with 3,500 GPH. Out of all the variables from each tank, the flow was clearly the most noticeable requirement for the fish.
Recently, when I changed the 180 gallon tank to house LPS corals and drastically lowered the GPH to a mere fraction of when SPS was in there it's habits seemed to alter. The fish started covering the entire span of the 180 gallon tank in search for the high flow zones that did not exist any longer. Where now that he is in the other 180 gallon tank ( in the same system ) where the SPS now reside, the fish seems more comfortable with residing in an area of the tank for a longer duration.
In the end I still feel confident in saying that the Achilles I have felt more comfortable in the 55 gallon with that extreme amount of flow than in the 180. So that aligns me with the thought process that
@shred5 has, along with all the other different variables that go into having a fish thrive in the environment.