Thanks very much for this! I have been trying to figure out for over a year now how to set a size limit on an APFS Time Machine volume.

Because I didn't know how to do it at the time, I created a TM volume on a 12 TB drive. In one year the size doubled from 850 GB to over 1.7 TB, so I knew I was going to be in trouble soon (I need most of the drive to be available for large files on another volume).

I successfully created a new TM volume with a 3 TB quota and have started using that for backups. The old TM volume is still there. I'm no longer using it for backups, but can still access and restore files form it if need be (I use BackupLoupe for that purpose). In a year or so when I need the disk space I'll delete the old TM volume.