SHRUNK! by F.R. Hitchcock
SHRUNK! follows Tom, who lives with his family next to the Bywater-by-Sea Model Village, as he discovers the ability to shrink things. He gleefully shrinks some sheep, a few boats and… the planet Jupiter. But then he loses it. With the *accidentally* miniaturised school bully shouting at him from inside his pocket, Tom has to find Jupiter and return it to orbit before asteroids pummel the village and the earth collides with the sun.