Configuring mailbox storage quota in Exchange 2007

From the Exchange Management Console

To set the default for the mailbox database for the company:

Server Configuraton > Mailbox > Storage Group > Mailbox Database > Right-click choose Properties > Limits Tab

To set it up for each individual mailbox (this overides the default for the mailbox database):
Recipient Configuration > Mailbox > Right-click on an individuals mailbox > Properties > Mailbox Settings Tab > Double-click Storage Quotas