I'm setting up dovecot (IMAP only) at an ad-agency (Lots of big files attached to mails). I'm wondering... What would you consider to be reasonable mail quota? My concern is, that some client-software will have problems with huge mailboxes. I was going to give everyone 2GB, but some users demand 15GB+. Does dovecot "prefer" mailboxes within a certain size?
I tend to think that it highly depends on the filesystem/IO scheduler etc, at least with maildir.
I'm using maildir on ext3 (with dirhashing) on a Xen instance (so I/O is terribly slow) and the limit up to which I can tolerate the wait seems to be at about 30-40k mails in a single folder irregardless of mail size. You can probably improve on that with XFS (directory listings are cached in memory rather than re-read every time) and by having a dedicated server, which I assume you do.
So if you have 30k 10MB emails you're already at almost 300 GiB :)
FWIW, my Inbox currently has about 16k mails with 276MiB size, but I'm fairly sure you can fare just as well with larger mails.
johannes