Storage Management for OWA-only mailboxes is available in CONTROL PANEL > Services > Mailboxes > OWA-Only Mailboxes > Storage Management or CONTROL PANEL > Services > Mailboxes > Disk Quota Management:

Storage Management > OWA-Only

On the OWA-Only tab, you can purchase additional disk space for OWA-Only mailboxes, manage Send and Send/Receive limits and notification settings. You can also choose to apply the settings to all existing mailboxes or new mailboxes.

Manage all mailboxes

You can manage each individual mailbox by clicking on its Display Name in CONTROL PANEL > Services > Mailboxes > OWA-Only Mailboxes and choosing Storage Management.

Manage individual mailbox