Oh those Frenchmen! Count on them to create a purely decorative enhancement to a fireplace. These single panel screens stand on little feet and were certainly invented by a host who didn't want his guests to view the ash created during the Bordeaux party the night before. Thoughtful, actually.
There is quite a bit of flexibility in determining the best size of decorative screen for a particular fireplace opening. The following information is intended as a guide only - there are other factors to consider such as scale of the room, hearth size, mantle size, shape and location, etc. Let your own judgment be your best guide -"beauty is in the eye of the beholder".
For a flat French screen there is a lot of flexibility - historically these were simply placed on the hearth as decoration and did not have to be the same width as the opening. So they can be larger or smaller than the opening in both width and height.
There is also flexibility in how tall the screen should be. Some customers prefer them 2-3" below the opening while others prefer 2-3" above the opening. We are not able to change the height of our patterns, but sometimes can vary placement of feet on a French screen or even add feet to a paneled screen for additional height.