A cool mist humidifier can not be used as a vaporizer, since it doesn't produce steam like the warm mist models and thus can not vaporize.
A cool mist humidifier is the most energy efficient, since it constantly adjusts its return of moisture back into the air; you can't over-humidify.
Sizes of humidifiers and air purifiers range from a tabletop size for a single room, to a large household unit that can humidify much of an average size home.