@iga
You've got the same Problem I've got when I had installed Recalbox 8 on an old Asus EeePC Laptop.
Duckstation was the only PSX-Emulator working, but I got only a 4:3 Picture
I've looked around everywhere but didn't find an solution to make it Fullscreen.
This
https://www.duckstation.org/wiki/Settings
is the "Settings" Page of the Duckstation-Wiki and here it says this
General Settings
Most of the settings in here you do not need to change unless you want DuckStation to start in Fullsscreen mode, enable the UI in Fullscreen mode or Enable Discord Presence.
Display Settings - Screen Display
Aspect Ratio is 4:3 for all PS1 games by default, most monitors are 16:9 as standard, you can enable the Widescreen hack in Enhancement Settings to correct the stretching/warping which also gives a larger view in 3D games though this may not work in all games, 2D games and games with pre-rendered backgrounds may not work as expected.
Well, I searched but I couldn't find an Access to the these "Enhancement Setttings" on Recalbox.
Maybe they only show up if you use Windows and install Duckstation as Windows-Version.
Here
https://github.com/stenzek/duckstation
it says
Linux
DuckStation does support Linux, but no support will be provided by the developer due to the huge range and variance of distributions.
But maybe someone from the "Recalbox-Staff" will show up and tell us how we can access the "Enhancement Setttings" of Duckstation or how we can change at last the Screenresolution.