The Teeny-Weeny Unicorn's Great Big Wish by Shawn Harris

Once upon a time, there was a teeny-weeny unicorn that wished to be big. But he never saw a shooting star, and a ladybug never landed on his arm. So when he comes across a dandelion poof, he decides to give his wish one last try. With a little help from a gust of lucky wind and a clever princess, the Teeny-Weeny Unicorn sets off on a quest to find something littler than he is.