Well, there's a lot of power ups, but I think he just absorbs most of them.
When Mario gets a mushroom, that's his normal size, if he got a super mushroom like that then he would be in his "Super Mario" state. He gets small because of poisen mushrooms or something.
That, or "Small Mario" is 5 feet tall and Peach is 12. Making Bowser about 30 feet tall.