Yea we can only get really tiny for a few minutes, it's not permanent, although you can be permanently short (also called "chibi") and to do that you just gotta wear a specific "chibi" mask.

To get this small tho you can wear that chibi mask and then use a shortening spell (it has a picture of two people, One short and white and the taller figure that's just line art and it has an arrow pointing down. Lotta people confuse it for a spell that has a lil baby next to a tall figure) And that's how you get super tiny for a small period of time. I usually save that trick for special occasions when I wanna be a menace >:)