For curtain width, the rule of thumb is to go 1 1/2 to 2 times the width of your window for maximum fullness. Curtain length is a personal choice. For example, if you're going for a more tailored look, a lot of people like the curtains to hang to the window sill. For a more elegant look, most people prefer the curtains to hang to the floor. If you want more light to come through your windows, you can mount your curtain rods above and past the sides of your window frame. That way, you can pull your curtain panels all the way over to expose your whole window.