I believe the actual size is included in the vault store database.I used the usage page (http://localhost/enterprisevault/usage.asp) to download the information per vault store, put that in excel, and then multiplied the archive size with 2 to be on the safe side. In theorie 1.5 is on average the compression rate, so 2 will give you a good idea of what to expect.
If you have many archives to do, I advise you to look at 3rd party tooling. I worked with Transvault, but Cloudficient is also a good one. Good thing of the last one is that you can migrate the archives directly to the O365 mailbox.
You can use below query as a starter. Ther comment line you can use to get items younger than x years
COUNT(S.ItemSize) "No. Archived Items",
SUM(S.ItemSize)/1024/1024 "Total Size (GB)"
FROM Saveset S,
WHERE S.ArchivePointIdentity = AP.ArchivePointIdentity
AND AP.ArchivePointId = R.VaultEntryId
AND R.RootIdentity = A.RootIdentity
-- AND S.IdDateTime < DATEADD(year, -5, getDate())
GROUP BY A.ArchiveName
ORDER BY A.ArchiveName