This can be not so hard to really hard.
If you need the size for the backups for images that are still active, then Heathe already posted the answer. If you need the sizes for images that have since expired, then NetBackup doesn't know about them any more and you need an extra-cost reporting tool like OpsCenter Analytics (for NetBackup 7 and up) or Aptare StorageConsole (my favorite).
For example, say you're doing daily incrementals with a 2-week retention, weekly fulls that you keep for a month, and monthly backups that you keep for a year.
The catalog will have the sizes of all of your backups for the last 2 weeks. NetBackup no longer knows about the incrementals that happened 3 weeks ago but knows about the weekly and monthly fulls. If you want to know what happened 3 months ago, you'll get a picture of the monthly fulls but nothing about the daily or weekly backups.
If you do purchase a reporting tool, remember that just buying it won't give you the picture of what happened 6 months ago. You'll get good trending history going forward.
.../Ed
