Ready-made curtains generally come in one of three standard drop lengths: 54 inches, 72 inches or 90 inches. A 54-inch drop should be level with the sill, a 72-inch drop should fall below the sill, and a 90-inch drop should reach to the floor.
There are also three standard widths of ready-made curtains: narrow (46 inches), medium (54 to 66 inches) and wide (90 inches). These measurements refer to the width of each ready-made curtain panel, which are usually sold in pairs. Ready-made curtain sizes are usually indicated in terms of width x length, so a 46 x 54 curtain has a width of 46 inches and a length of 54 inches.