System Settings --> Appearance --> Cursors offers 3 sizes: 24, 36, 48.

24 is too small and 36 is too big. Is there a file somewhere that I can edit to change the size to something I want?

All of the googling I've done has lead me to editing it in Gnome, or to just use the GUI.

I've found this but it doesn't seem to work:
It says to go into home, create a file called ".Xdefaults" and fill with "Xcursor.size: size"
https://beamtic.com/kde-huge-cursor-size

Thanks!