Actually, some triggers get huge, but not all. Stay away from the Queen trigger...not only are they super aggressive, but they grow fast. Triggers that don't grow as fast include the Huma, Blue throat, Niger and Pinktail to name a few. You can keep 1 of these in a 55 gallon, but they should be the last fish added to the tank and beware that they will fight with other tank mates.
My experience with triggers is that you want a bigger tank just to limit the aggression of these fish. The ones I mentioned above grow like a football and probably won't outgrow a 55 gallon for at least 2-3 years, if you get them at an initial size of 1-2 inches. I've had a huma and blue throat for about 5 years now and besides them being super aggressive, are great fish to have-very hardy and resistant to disease and are virtually indestructable.