Texture resolution is recommended to be kept at 4K max and to utilize the compression menu to optimize these files. Ideally, use a max of 3 Shader networks; PBR Metalness/Roughness is our standard workflow.
Each 'Shader' may have 1 to 5 .png maps composed of a Diffuse, Metalness, Roughness, Normal, Emission, and/or Opacity (use ORM as often as possible).
Change your materials from Standard to Physical in the engine when using Glass or Car Paint Shaders. To understand more of this process feel free to read up on three.js materials here: https://threejs.org/docs/#api/en/materials/MeshPhysicalMateria