Hi,
I don't know of any way in the current release, the next release (6.3 - 2006Q4) is supposed to offer per-folder quotas e.g.
To specify a 2 Gigabyte quota; a 20 Megabyte voice mail quota; and a 100 Megabyte quota for the Archive folder:
mailQuota: 2G;#voice%20M;Archive%100M
The two gigabyte quota represents all folders in the user's mailbox that are not explicitly assigned quotas. In this example, that excludes messages in the Archive folder, and messages of type voice. The 100 Megabyte quota includes messages in any folders within the Archive folder.
Regards,
Shane.