I briefly saw the
Next Backup Size entry on my phone deep in the Settings somewhere, but not I cannot find it at all.
Where exactly is it?
Settings → Apple ID → iCloud → Manage Storage → Backups → This iPhone/iPad.
This location is documented at https://support.apple.com/en-us/HT204247 in the section titled Choose which apps to back up but doesn’t spell out the next size calculation.