Can you set the panel style to be the French Panel. This one allows you to then set the width of each section of frame, (top, bottom, left and right) and you can split them with a grid. Just set that to 1 horizontal and 1 vertical and you’ll get a single glass pane at the size you want.
ArchiCAD 4.5 - Archicad 26UKI Apple Silicon 4032
macOS Ventura (13.2)