Determine the disk space used by a Time Machine backup
Usage: sudo time-machine-size /Volumes/MyBackups/Backups.backupdb/MyVolumes/2012-01-01-000000
Time Machine creates backups using directory hard links, so it's not obvious how much space will be saved when a backup is deleted. This tool prints out all the files that exist in only this particular backup, and finally the size used by the backup, in MB.
Note that finding the size of two backups individually and adding them together will not accurately determine the size you'll save by deleting them both. Some more space may be saved than expected, because of files present in only the two given backups and none others. If this matters to you, run time-machine-size again after deleting the first backup.