If its purpose is purely decorative and you intend never to roll it down, you can use a short remnant of fabric. To protect your room from the sun or a neighbor's view, use a sheer, lace panel or a sun-filtering shade. Of course, such fabrics become transparent when the lights are on. The choice is largely aesthetic, but you can first base it on the window's style, scale and condition, then the room's scale and style. Shades, blinds and shutters are geometrical and can draw attention to unshapely windows.
Curtains are more forgiving.
You can use a combination of treatments if you have an elegant, well-placed window. Use inside measurements for treatments that go inside the window frame. Measure across the narrowest point of the frame, because an old window may be uneven. To use an inside treatment, your frame needs to be at least 1.5 inches deep. A suggested width is 1.75 inches past each side of the window.