Graphic levels usually refers to the amount of textures used ( vram), the complexity of the shaders rendering the scene and stuff like shadows, number of pixel lights, AA etc. In which case EqualsEquals answer is all you need.
Its very rare, if ever that you provide distinct resolutions for models. Most of the time a single high quality model is used and will be dynamically LOD in the scene if polygon count is an issue. Not sure if Unity supports that though, in which case providing distinct resolutions may be beneficial, but I highly doubt you'd need to, as its a pretty specialised requirement.