So much problem with screen.width camera.pixelwidth and responsive design

I do not understand how to make the gameobjects responsive to the different screen size.

Here the platform is long finite and camera is set to perspective, how can i make this cube width responsive so that on the start of game the width adjust to the width of the phone screen.

Is there any way i can control the width, height, dimension of gameobjects with SCRIPTS !!! transform.localscale scales the width and behaves in a weird way.

i have tried using screen.width height and worldtoscreenpoint,screentoworldpoint but could not understood how to make this work.

Please help