Hey @LeoKSimon
This isn’t really a Fvwm problem, believe it or not. What you’re asking is all about HiDPI scaling, which is aimed at GUI frameworks for applications (such as GTK/QT). The best advice I can suggest is here:
https://wiki.archlinux.org/index.php/HiDPI
Ultimately, the state of HiDPI support on Linux is pretty poor, outside of applications which aren’t using GTK/QT.