Hi Megan,
Most AV's are given a size by the hybridizer when they register them. According to AVSA minis are up to 6 inches, semis to 8 inches standards are over 8. The national does not as yet recognize a micro mini so they fall into the category of minis. The sizes are determined by the hybridizer when the plant reaches its optimum growth. Trailers are designated semi, mini and standard usually by leaf size. This is also another way to tell what size a plant is. However, many hybrids in the mini and semi classes are not always correct. Some hybridizers give a designation to a mini and just keep it small by removing leaves. Also there are many semis that should have been designated as small standards. So the true test of a mini is to grow the plant to its optimum size and then determine it's class. As for micros, many people call them micros and keep them in tiny pots to keep it small, it's sort of like what the Japanese do to bonzai a plant. But there are a few that are really tiny and could fit into a micro class if there was one.
Sorry to be so ambiguous about it but thats about the way it goes.
Fred in NJ