Steven,

 

Brian Ellwood is correct.  For a detailed explanation:  as a domain admin, go to Settings | Domain Settings | Advanced Settings | Default User Settings.  On the User tab you can specify the Mailbox Size Limit (in MB).  Make any of default user setting changes and save. (this will be applied to all new users that are created). Next on the User Propagation on the Options tab, make sure All Users is selected.  Go to the User tab and check the Mailbox Size Limit checkbox.  Press Propagate Now and domain users should now have their limits set to your specification.

 

I hope this helps.