I’m familiar with making shape keys to hide a object in blender you just do it in edit mode, but how do you use shape keys to resize a tail with the bones and not just the mesh?
Shape keys are setup on the 3d model itself, not in unity.
Unless the person that made the model made a shape key for the tail, there won’t be a shape key for it in unity.
If you want to add shape keys to certain body parts, I suggest you grab blender and grab a copy of cats GitHub - absolute-quantum/cats-blender-plugin: A tool designed to shorten steps needed to import and optimize models into VRChat. Compatible models are: MMD, XNALara, Mixamo, DAZ/Poser, Blender Rigify, Sims 2, Motion Builder, 3DS Max and potentially more
And modify the avatar itself, you can use the pose tool in cats to adjust the tail bone and turn it into a shape key.