Let me know if I understand correctly: since the glTF 2.0 specification specifies meters as units for storing geometry, you would expect the External Display component to take this into account, as well as the units specified in your Grasshopper model (using the Tolerance Settings component), in order for the viewer to scale the glTF in the scene from their native meter unit to the unit you specified.
Does that sound like a sensible way forward?
Alternatively, we could just add an input to the External Display component that allows to specify a scaling factor.