Customizing Size Category

[Trester]

Would there be a way to make something smaller? In customize I can make the size of a player character larger by using the Customize>Size Category, but not a way to make the creature smaller.

Reason for asking, a friend wants to make an Uplifted Bear, but he would like it to be a Red Panda, so I would need to take the size from Large Magical Beast to Small Magical Beast.

If there is a way to do this please let me know.