We will basically need only two measurements from you for an arched window. Height and Width.
In addition to the Height and Width measurements, you will need to take a measurement for Side Height. Side or Bend Height is the distance from the bottom of the window to the start of the bend of the eyebrow. See the picture below for more details.
Window Screen Resource Center
The resources below provide information and articles meant to educate and answer any questions you may have related to window screens.