Is there a config file to change font sizes system-wide?
There's the user-specific kdeglobals file but I couldn't find it among the global config files.

systemsettings -> app appearance -> fonts

google01103 wrote:systemsettings -> app appearance -> fonts

No, I want to change the font size for all users.
That would just change it for the user currently logged in.

In this case you probably need to create a system wide kdeglobals file in the appropriate directory, filled with the appropriate entries. This will then be cascaded down to all users who have not set their own font settings.

The appropriate path to add these system wide files into is usually the last path mentioned in the output of "kde4-config --path config" which is "/etc/kde4/share/config/" on my system.

It is said here (https://unix.stackexchange.com/question ... ngs-stored) that since plasma there is no such default location. You put whatever you need into /etc/skel/ and it will be copied for all newly created users.

For instance if you have a file /etc/skel/.config/kdeglobals, a new user will have a file /home/newuser/.config/kdeglobals