Run your game, put on the hat … goto Explorer and find your player then the hat.
There are one of two things that can change the size, the handle Size or if there is a mesh within it, then it would be Scale. Figure out how to get to that and change it to what you wish.
Also the input would be a Vector3 … So they put on the hat, then you change it’s size.
If you wish this to be the way it is always … change the mesh Scale.