What is the Opening Size (or Viewing Area) on a Fireplace or Fire Box?

The Opening Size of a fireplace or fire box describes the dimensions of the viewing area. In other words, the Opening Size does not include the overall dimensions of the product nor the outer frame or flange surrounding the viewing area, but only the dimensions of the innermost box, into which you gaze happily at the dancing flames of your fireplace. The fire box in the image below shows an Opening Size, or Viewing Area,of 68.1".