API or object for getting user mailbox size/usage?

Version 1
    This document was generated from CDN thread

    Created by: David Roberts on 10-12-2012 06:36:51 PM
    hi,
     does anyone know of a REST API object and/or performance counter which can tell me the used mailbox size for a user?  I'm trying to proactive detect when users are nearing their mailbox limits. 
     
    Thanks