this has been broken for a while - long story short the DOH interface that the back end SA goes through to pull information is only getting directory information and not messagin information such as mailbox limits/size and so this field just remains unpopulated.
It'll be removed from the SA, probably in the next release - you can use the Message Store Manager to get mind-numbingly details views of your users mailbox information including limits, what types of messages are there, how long they've been there etc... you can check it out here:
http://www.ciscounitytools.com/App_MSM.htm
