There is no specific Minecraft mod that lets you change your size. However, there are a few mods that allow you to change your character's model, which in turn can change your character's size. For example, the Player API mod allows you to change your character's model, and the Smaller Player mod allows you to change your character's model to that of a smaller player. Alternatively, you could use a resource pack to change your character's model to that of a smaller player.
