Display

void Start()

{
    // GUI is rendered with last camera.
    // As we want it to end up in the main screen, make sure main camera is the last one drawn.
    extCam.depth = camera.depth - 1;
    camera.SetTargetBuffers(Display.main.colorBuffer, Display.main.depthBuffer);
    extCam.enabled = false;
}
void Update()
{
    if(Display.displays.Length > 1 && !extCam.enabled)
    {
        Display.displays[1].SetRenderingResolution(256,256);
        extCam.SetTargetBuffers(Display.displays[1].colorBuffer, Display.displays[1].depthBuffer);
    }
    extCam.enabled = Display.displays.Length > 1;
}

SetRenderingResolution

Rendering Width.

displays
main