Four sizes, actually, it's just that the US doesn't recognize the 'medium' (between mini and standard, also called moyen, klein, mellom, etc.) for some reason. FCI does though.
Poms can be great little dogs! They tend to be barkers - a lot of the little guys were originally bred in part to alert to unusual things - and that fluffy coat needs regular care to keep it looking fabulous. Like all small dogs, they need to be trained just like big dogs and have their emotions respected. Unfortunately a lot of little dogs get grabbed, picked up, etc. because it's easy, and are never allowed to say 'no' when this makes them uncomfortable, which is why they tend to get a reputation for being nippy and having attitudes - it's the only way they've found to get people to stop.
Agree with avoiding anyone advertising 'teacup' or 'micro' dogs. These breeders are either liars (often they sell the puppies too young but claim they're 8 weeks so they look smaller) or breeding unhealthy dogs to get those tiny puppies. Heartbreaking, really, because they care more about slapping a crazy price tag on then the lifelong health and happiness of their dogs.