You can add the "-forcedesktopscaling" variable to your startup, specifying a numeric value.
For example, for Fedora/KDE Plasma I have the 'Command-Line Arguments' field, for the Steam entries, in the 'KDE Menu Editor' app, to say "%U -forcedesktopscaling 1.25".
The old GDK_SCALE variable is no longer being looked at by the Steam client (as far as I can tell, and in case you were using that variable).