This is a quick blurb from Robert J Fenner's book The Conscientious Marine Aquarist
"The Yellow Boxfish (Ostracion cubicus) is a perennial favorite of mine, especially as a bright yellow juvenile with black spots. It is a large fish in the wild, attaining some 18 inches. On a smaller scale, the Blue-spotted (male) and Black-spotted (female) Boxfish, (O. meleagris) is often seen as well. The so called "Boston Beans" that turn up seasonally in the fish shops are juveniles of various species of cowfishes and boxfishes, not a single species; they grow into a variety of adult forms."
From personal observation and experience, I can tell you that I agree with Robert Fenner. I have seen HUGE boxfish/trunkfish/cowfish in the wild, and have kept many species in captivity. They are very long-lived, but grow very slowly because of the nature of their bony platelike skeletons. Assuredly, given enough time, proper care, and adequate calcium levels in the water, they will get big (that is, of course, if it is a species that can do so)