Hello Sharonkim, here is an extract from the German instructions:
The SureFlap pet flap is ideally suited for installation in single and double glazing. If this requires cutting an opening in a pane, a specialist glazier must be engaged as this requires specialist knowledge. For the opening in a pane, the ideal diameter is 300 mm. A mounting adapter is available to complete the installation in glass panes. This covers openings from 294 mm to 320 mm diameter, as shown in figure 10.