Standard Doors tend to have wider frames that allow one size to fit a variety of opening dimensions. Using your measurements, find the correct door by finding where the dimensions fall on the chart for both the width and height of the fireplace opening.
Standard doors are typically associated with a model number, as we see here on the left of the chart.
Custom Doors require full measurements be taken of the fireplace and each door is uniquely drawn to fit that specific opening.
As such, many custom doors offer thinner frames and larger glass.