@warwar If you are using an Rpi, you can change the videomode per emulator in the section:

------------ I - EMULATORS CHOICES -----------

But if you are using a computer, x86 or x86_64, you will need to use the xrandr command.

Read:
https://forum.recalbox.com/topic/14398/menu-and-kodi-is-not-full-screen-on-old-netbook/4
https://github.com/recalbox/recalbox-os/wiki/Change-display-resolution-on-x86---x86_64
https://github.com/recalbox/recalbox-os/wiki/Video-configuration-detailled-(EN)
https://elinux.org/RPiconfig