Start by taking measurements of your window, so you can choose curtains. A good rule of thumb is to get curtain panels that are twice the width of your window or even slightly larger, so you get luxurious folds when they’re hanging and full coverage when they’re closed.
The curtain length you choose depends on how high you’ll be hanging your curtain rod and how close to the floor you prefer your curtains to be. A good rule of thumb is 4 to 6 inches above the window frame, but to make a room feel taller, hang them closer to the ceiling, up to 12 inches above the window frame. For a clean, casual look, choose a length that falls half-an-inch above the floor. For a more luxurious puddle effect, choose curtains that are 6 to 8 inches longer than the rod to floor measurement.
Don’t forget, the header of the curtains will factor into height measurements. Rod pockets will hang shorter than grommets or tab tops.