How can I change the size of a SpecialMesh to the Size of its parent?

Let me explain. A Specialmesh contains the value Mesh.Scale Rather than Mesh.Size. By default, Scale is set to 1,1,1. I am trying to get the size of the SpecialMesh to match the size of its parent, but I can't find an absolute scale for any of the meshes because each Mesh is a different size.

Part.Mesh.MeshId= "rbxassetid://000000" --just an example
Part.Mesh.Scale = Part.Size -- This doesn't work

Saying Mesh.Scale = Part.Size will not correctly change the size of the SpecialMesh. Is there any way to do this?

Make the SpecialMesh a MeshPart, where you can scale it correctly.