Look for event ID 1221 in the application log, this will tell you how much free space there is.
Also have a look at my article here for some additional information: http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_2730-Exchange-Offline-Defrag.html
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In this video we show how to create a mailbox database in Exchange 2013. We show this process by using the Exchange Admin Center.
Log into Exchange Admin Center.: First we need to log into the Exchange Admin Center.
Navigate to the Servers >> Data…