They get bigger than the Fishbase listed size. Still, a 5-6" is quite big in respect to a 40 breeder or 75 gallon.
I just looked through you're 180 build and it looks like you had a larger Acanthurus nigricaudus and a Acanthurus nigroris tang. Both of which seemed to be in the general 5-7" range. Could you imagine either one of those in a 40 gallon? Happy/Healthy butterflies like to be out and about, swimming. They aren't stationary fish by any means. They're certainly not perchers or sand dwellers.
Better question. Would you keep a 4-5" Sailfin Tang in a 40 Breeder? Heniochus swim just as much as Zebrasoma tangs, granted at a slower pace.