To make an avatar have body scaling, the blender model must be imported to Unity with blendshapes, the size you can choose from to scale body parts depends all on these blendshapes, now if the avatar has these already you just need to make a controller and a couple animations (easier to do than it sounds) in Unity
Now for SPS its even easier, you´d need to have the SPS and VRCFury imported onto your Unity project, there is this VRCFury tool that you can set to an object (in this case the peen) and it will automatically set a Plug to make it work with SPS holes