A fully-grown adult Copperband butterfly will grow to between 7 and 8 inches (17.8 and 20.3 cm), although different sources will specify fairly specific maximums within that range:
Angelfishes & Butterflyfishes (Michael 2004) - Lists maximum size as 7.8 inches (20 cm)
Marine Fishes (Michael 2001) - Lists max size as 7.9 inches
Live Aquaria - Lists max size as 8 inches