It looks like Zimbra has an easy way to report message count per folder (zmmailbox getAllFolders), but not folder size.
Size information is buried in the database, and needs a few queries per folder to dig..
Here is a script to do it for you.. zimbra-size.sh
[zimbra@zimbra ~]$ /tmp/zimbra-size.sh sakis@zimbra.gr
sakis@zimbra.gr's max mailbox size = 20480 MB, current mailbox size = 343.02 MB.
size (MB) msgcount unread folder
--------- --------- ---------- ----------------------------
0 0 0 /Chats
0 0 0 /Drafts
12 221 221 /Inbox
15 1001 1001 /Inbox/Tickets
310 7158 7158 /Inbox/Syslog
3 78 78 /Inbox/Wiki
0 0 0 /Junk
3 21 0 /Sent
...