A 1.5" rod will do the trick. I would talk to the workroom making your draperies. For a window that large, you will need a rod with at least one center support, maybe more than one depending on the weight of the fabric and the lining you've chosen. There are some rods that have mounting brackets at the top so the movement of the draperies is not obstructed by a bracket. This is the type of rod I would recommend based on the width of the window and likely weight of the fabric.