Find the size of a given Time Machine backup
There is no such thing as a given backup image in Time Machine. Practically speaking only changed files get backed up each time but the snapshot volume also contains links to the most recent version of all unchanged files on the backup drive. So each snapshot volume represents a backup of your whole hard drive (also summing up the size of all snapshot volumes together will give you a number significantly higher than the size of your whole backup disk).

To find out how much data was added/changed/removed during one Time Machine backup, tmutil calculatedrift /Volumes/Time\ Machine/Backups.backupdb/NameOfYourMac is the way to go:

pse@Fourecks:~$ sudo tmutil calculatedrift /Volumes/Time\ Machine/Backups.backupdb/Fourecks
[...]

2013-06-02-131853 - 2013-06-02-164246
-------------------------------------
Added:         807.2K
Removed:       5.0M
Changed:       100.7M

2013-06-02-164246 - 2013-06-02-174230
-------------------------------------
Added:         9.9K
Removed:       0B
Changed:       43.8M

Drift Averages
-------------------------------------
Added:         715.6M
Removed:       535.5M
Changed:       571.5M