The only known trigger to stay under about 8'' was the "Xenobalistes" Outrigger Triggerfish, a truly unusual animal. Unfortunately, this animal turned out to be just a baby Blue Throat! I guess that explains why so few were ever found.
The smallest triggers are certain Sufflamen triggers if I rememeber correctly, and these could be placed in a 55 gallon tank. Other species could be maintained in a tank this size when they are babies.
-Lynden