In general, your curtains width should be double the width of your windows. For length, this depends on if you want floor length, sill length curtains or below sill. For sill length curtains, they should sit 1cm above the windowsill. Below length curtains generally sit around 15cm under the sill. For floor length curtains, ensure there is at least a 1cm gap between them and the floor. If your window has a radiator underneath it, the curtains should stop around 2-3cm above the radiator to avoid heat being absorbed by the curtain.