Old EV6 for decommissioning.

What version are you?  If running EV 10 sp2 there is a nice archiving report created after the schedule completes.

As for Retention categories, you could use this sql, it will run against the Vault Store database and you need to place the name of the archive where it says Your ArchiveName.

If you comment that line out it will run against all achives but may take some time.

SELECT A.ArchiveName
       ,CAST(AF.FolderPath AS NVARCHAR(MAX))
       ,AF.FolderName
       ,COUNT(S.IdTransaction) "Number of Archived Items"
       ,rc.RetentionCategoryName AS "Retention Category"
      -- ,SUM(S.ItemSize) "Total Size (KB)"
FROM   Saveset S,
       ArchivePoint Ap,
       Vault V,
       EnterpriseVaultDirectory.dbo.Root R,
       EnterpriseVaultDirectory.dbo.Root R2,
       EnterpriseVaultDirectory.dbo.Archive A,
       EnterpriseVaultDirectory.dbo.ArchiveFolder AF,
       EnterpriseVaultDirectory.dbo.RetentionCategoryEntry rc
WHERE  S.VaultIdentity = V.VaultIdentity
  AND  S.ArchivePointIdentity = AP.ArchivePointIdentity
  AND  AP.ArchivePointID = R.VaultEntryID
  AND  V.VaultID = R2.VaultEntryID
  AND  R.RootIdentity = A.RootIdentity
  AND  R2.RootIdentity = AF.RootIdentity
  AND  rc.RetentionCategoryIdentity = s.RetentionCategoryIdentity
  AND  A.ArchiveName = 'Your ArchiveName'
GROUP BY A.ArchiveName, CAST(AF.FolderPath AS NVARCHAR(MAX)),AF.FolderName, rc.RetentionCategoryName