Physics asset problems and character flies into space!

I’m using the ALS system for ragdoll. My character goes into it when falling from x or y height, then goes back to normal when impacting the ground when using the standard mannequin.

After many problems, I’ve managed to get a character I like from Mixamo, converted it with Blender 2.8 so it has all the right names for the bones and convert the blend space. BUT when I get to the physics asset, it’s way too small for all the bones, so the standard mannequin one won’t work. I’ve had it auto setup, highlighting the bones I want connecting, then adding the constraint. Then we get to the Root one, and it’s way too big. I can’t make it any smaller. If I don’t have a Root, when the character comes out of ragdoll, it gets stuck. If I do, then sometimes it flies off into space!

Can someone please tell me how to scale it so I can add the small Root so it won’t go flying anywhere? Or point me in the direction of some alternative?