Provider would like to limit the maximum size of an Exchange resource mailbox that customer may create in POA Control Panel.
There is no separate limit for Exchange resource mailboxes. A more generic limit for all mailboxes also applies to resource mailboxes.
This limit is governed by the 'Maximum Allowed Mailbox Size' sub-resource of the main 'Hosted Exchange' Resource Type:
It can be managed either on a Service Template/Service Plan level or at subscription level, in the same manner as other resources.
For example, if you want to alter the limit for a particular Service Plan, you can include a resource rate for this resource in PBA with the desired amount set as 'Included'.
A feature request with ID POA-78228 exists to improve this functionality. Please contact your account manager regarding its status.