On the cli you can get a count on every folder in an account like this:

[zimbra@zimber tmp]$ zmprov gmi a.geezer@domain.eu
mailboxId: 339
quotaUsed: 8978573367
[zimbra@zimber tmp]$ zmmailbox -z -m a.geezer@domain.eu gaf >/tmp/a.geezer.stats
[zimbra@zimber tmp]$ mail -s "a.geezer stats" admin@domain.com </tmp/a.geezer.stats