Another handy little magic spell for MacOS Sonoma’s Time Machine tool is…

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info | grep "Estimated full backup will"

…which – when entered into a suitable Terminal instance – will print out Time Machine’s recent estimates of the storage costs of a full backup…

2024-05-02 06:46:35.507588+0100 0x3c5be8   Info        0x0                  291    0    backupd: (TimeMachine) [com.apple.TimeMachine:SizingProgress] Estimated full backup will contain 3113124 files (8.91 TB) from all sources
2024-05-02 07:46:49.802454+0100 0x3cd657   Info        0x0                  291    0    backupd: (TimeMachine) [com.apple.TimeMachine:SizingProgress] Estimated full backup will contain 3113163 files (8.91 TB) from all sources
2024-05-02 08:46:57.526626+0100 0x3d611b   Info        0x0                  291    0    backupd: (TimeMachine) [com.apple.TimeMachine:SizingProgress] Estimated full backup will contain 3113223 files (8.91 TB) from all sources
2024-05-02 09:47:03.731963+0100 0x3e04eb   Info        0x0                  291    0    backupd: (TimeMachine) [com.apple.TimeMachine:SizingProgress] Estimated full backup will contain 3114585 files (8.91 TB) from all sources
2024-05-02 10:47:35.409635+0100 0x3ecb04   Info        0x0                  291    0    backupd: (TimeMachine) [com.apple.TimeMachine:SizingProgress] Estimated full backup will contain 3116304 files (8.91 TB) from all sources

All very useful when shopping for a new target disk of if your monitoring your storage growth.