Hi,
Since the preview asset inherits the transform, I would recommend creating a new Character Blueprint and bringing your character in there by adding a skeletal mesh component. You can then add a static mesh component and make it the GenericHMD mesh. This will show up in editor though, but could always be removed if you just needed it for reference. This is a way of attaching it to the character and avoiding the mesh itself inheriting the transform. From there you can have the Character Blueprint use the Animation Blueprint(can change this in the Details panel of the editor) that you want and use the Transform(Modify) Bone Node in. This will let you scale the (in this case head bone) bone to the size you want without affecting the transform of the GenericHMD mesh. This should give you the freedom to change the bone you want and then scale the GenericHMD mesh to be the size you want. Let me know if you have any questions about using this setup.
-Thank you