Thumb Princess (1999)

Saeko Machida is a high school girl with a crush on a boy in her class named Yuichi Kimijima. Becoming obsessed with him, she tries to gain his attention and win his affections, but there's no such luck. One day, she meets a strange man at a flea-market who gives her a red liquid, telling her that if she splashes it on Yuichi, her wish will become true. She acquiring the liquid, certain that the liquid will cause Yuuichi to love her. Saeko finally gets the courage to confess her love to Yuichi, but he rejects her. Disappointed, she splashes the liquid on Yuichi, but that liquid actually causes him to shrink about three inches in height.