How to scale the size of the displayed screen?

Hi
we use Nextcloud Talk, version 30, with Firefox browser on Win11. The members of a meeting have all the same screen sizes, and when user A shares his screen, then his screen-size with 100% shrinks to fit to my screen - 95% screen content of the other user and 5% invisible frame, caused by my browser. The browser is set to “fullscreen display”, but nevertheless it has a small, unused border.

To watch a screen shrunk to 95% quality is not funny for my eyes…

So how can I set the software to display an unscaled screen with 100% quality?

Thanks for advice!

Edit:
Will be continued in German forum:

Thanks