Fully grown, healthy triggers can grow anywhere from 9 - 20 inches. Smaller species of triggers are the Bluejaw trigger, Bursa trigger, Picasso trigger - all reaching around 9-10" - minimum tank size should be 75g or larger.
Larger species ( up to 20") are the Niger trigger, Blue Lined trigger, Clown trigger, Queen trigger - all these triggers require large tanks when fully mature - 125g or better.