The Incredible Shrinking Girl by Lou Kuenzler
Violet is normal-sized one minute and pocket-sized the next! It's the BIGGEST day in Violet's life. All she has ever wanted to do is ride Plunger, the scariest roller coaster around. And today she is finally TALL enough. 1.4 meters to be exact. But just as Violet is about to climb into one of Plunger's carriages, something totally crazy happens. Violet shrinks! She is suddenly as small as a fish finger. Although Violet doesn't stay little for long, her chances of ever riding Plunger are ruined. She never wants to shrink again...But then Granny is accused of stealing, and tiny Violet might just be the only one who can catch the thief. The first book in a fantastically funny new series!