what is the standard width of a curtain panel?
It can vary from 50″ up to 58″, and some fabrics, especially sheers, can some in double width at 108″ wide. But the standard in the U.S. is 54″. So, if you think about a regular size window which is 30″-36″ wide, one width of fabric is only 20 or so inches wider than the window.