I would agree with the 125 being the absolute minimum for the smallest species (Naso literatus, which can reach 18''). They are large and fast fish. They are robust enough to be kept with very aggressive fishes, but gentle enough to be kept with even small Gobies. They also have two pairs of caudal spines, which cannot be tucked into a groove like most Acanthurids, and pose a signifigant danger to the aquarist if they are cornered. They will not attack unless severely provoked; thus, if a Naso attacks someone, the person probably deserved it.
-Lynden