I'm a recent Windows to 2021 macBookMacBook Pro convert. Not knowing it was impossible, I managed to stumble on a method using Disk Utility.
- Launch Disk Utility
- Erase the disk and create “Time Machine Backup” volume
- Notice that no option to limit size is available.
- Delete the Time Machine Backup volume just created.
- Add a volume named Time Machine Backup and supply size constraints.
- Add additional volumes with or without size constraints.
- Launch Time Machine and Select Disk ->Time Machine Backup
- Here's what I ended up with. Seems to work.