If you have a rod

Length: Measure from the top of the rod to the desired length you wish your curtain(s) to cover.
Width: Measure the width of your rod between the finials.

If you don't have a rod

Length: Measure the desired length you wish your curtain(s) to cover.
Width: Measure the width of the area you wish your curtain(s) to cover.

If you have a rod

Length: Measure from the ceiling to the desired length you wish your curtain(s) to cover
Width: Measure the width of your rod between the finials.

If you don't have a rod

Length: Measure from the ceiling to the desired length you wish your curtain(s) to cover
Width: Measure the width of the area you wish your curtain(s) to cover.