Comment 11 for bug 1770948

I "fixed" this on Cinnamon (Ubuntu 18) by switching away from unity-greeter:

$ cat /etc/X11/default-display-manager
/usr/sbin/lightdm

$ sudo apt-get install lightdm-gtk-greeter

then create /etc/lightdm/lightdm.conf with:

[SeatDefaults]
greeter-session=lightdm-gtk-greeter

This also somehow avoided the buggy 4096x2160 resolution that my TV crops.
   4096x2160 24.00 23.98
   3840x2160 30.00 25.00 24.00 29.97 23.98
   1920x1080 120.00 100.00 119.88 60.00* 60.00 50.00 59.94 30.00 25.00 24.00 29.97 23.98

unity-greeter picks 4096x2160@24 and triggers the large cursor issue (desktop, window decorations, taskbar etc)
lightdm-gtk-greeter picks 3840x2160@30 and the cursor is working correctly everywhere I checked