It is really quite easy. Choose the size that best expresses your concept.
All those reasons that the fellow gave you are minor surface reasons compared to the real one: People are largely printing big (and/or printing alternative processes) because everyone and their dog can take a technically sound picture with the highly-sophisticated equipment that is around for peanuts these days. They are pulling out all the stops to make themselves stand out as "real artists".
In short, it is an attempted and/or actual elitist thing...and I mean the word "elitist" without connotation.