Was it a silly question, why you can set quota on users,. but not on ibays??
As you've already been told, quotas apply to full partitions only, not directories. And i-bay is a directory on the file system.
Quotas can apply to users or to groups. SME server's server-manager only applys quotas to users, but it's possible you could use group quotas to restrict the size of ibays - or rather, the size of the set of ibays which share a group id.
If you use quotas, then the size of an ibay is limited to the sum of the quotas for all the users who have write access to the ibay.