With your setting, you expose the faces of the mesh.
If you model your mesh with that in mind, you will have a perfect control of the size of each face because you will build them in your modelisation software.
In deed, you’re looking for some sort of anti-tesselation system
It would be a bit dysfunctional to artificially reduce the number of poly by material on a too high poly model, no ?
You build a normal map from the world position of the faces in your material.
In fact, you just remove the smoothing effect in the lighting of each poly… It’s the same ting as the “facets” render style in 3DS max.
So, what you see is the polys of the mesh , without smoothing lighting