Okay kiddo, imagine you are holding a magic wand. If you wave it, you can change the size of anything you want, like making yourself as tall as a giant or shrinking down to the size of a mouse.
In some stories, the characters also have this kind of magic power. Sometimes they can change size to fit through small spaces or to fight against big enemies. This is called size change in fiction.
It's like playing make-believe with your friends, where you imagine yourself as a superhero or a tiny person exploring a big world. It makes the story more exciting and fun to imagine different sizes and how they would affect the characters and their adventures.