Kinda learning blender for VR Chat as I go, I have a model I've been testing stuff with until I have a good feel but it says in the documentation that the model "has three different sizes of leg and hip options as shape keys and you can use them to alter the size"

This is alien to me, I looked up key shapes but what I read doesn't really match what I see in blender. How exactly do I use these things to change the shape of the model?