Always measure the curtain track or pole width, not the window. If fitting a new track or pole, this should extend 6 inches to 12 inches either side of the window (to allow the curtains to be stacked back off the window) and 5 inches to 6 inches above the window recess.
Measure between the decorative ends-C
When measuring for curtains please use a metal tape measure as cloth or plastic tape measures may stretch. Always quote sizes in inches.