I tried with Cinammon desktop and searched for “lightdm” which took me to System Settings → Login Window.
I went the the “Settings” tab here and set HiDPI support to “Enable;” this fixed the mouse tracking speed and also made the password box larger.
Next, under the “Appearance” tab, I doubled the “mouse pointer size” from 24 to 48. This fixed the pointer size.
I noticed that the date/time changed on the /etc/lightdm/slick-greeter.conf file had changed so I looked at the contents.
This is what was written:
[Greeter]
enable-hidpi=on
only-on-monitor=auto
cursor-theme-size=48
I will mark this as the “solution” but I really wish there were an easier way to do this within KDE.
I don’t understand why KDE doesn’t have any way to edit these settings and why it configures SDDM rather than lightdm (correct?).