The Avus screen is 89,7 x 138mm and a 9x12 plate is 89 x 119.

From the manual:

In case the focussing screen should get broken take particular care to remove all splinters of glass from inside the camera. The screen frame is taken in both hands with the screen turned to oneself, and by means of the thumbs the broken screen is pushed away to the open side of the frame. A new focussing screen is easily inserted in a similar manner.

the focussing screen has a smooth shiny side and a grained side (matt). This is the side which should always point to the lens. In extreme cases when no new glass can be obtained an ordinary unexposed photographic plate will serve temporarily very well for the purpose. Here the emulsion side must face the lens.

Always see that the corners of the glass are slightly cut away to allow an escape of air out of the bellows when closing the camera.