Usually, when you purchase a pet door, you cut a hole in a door to fit the pet door. We can help. Usually this situation comes up with French doors. You want to remove a pane of glass and install a pet door in its place.
Some french doors are a large sheet of glass with wood facing to look like separate panes.
That is a very different situation and one to be avoided if possible. Check that the resulting pet door will be large enough for your pet. You'll lose two inches in both dimensions. If necessary, you may need to remove more than one pane of glass. Also, you can cut down into the wood cross piece to make the flap taller if necessary.