I would recommend four ways of doing so:
1. Exchange Server Profile Analyzer tool: Will analyze all logs, etc. and will provide average mailbox size. This is a free download from MS at http://www.microsoft.com/en-us/download/details.aspx?id=10559
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To show how to generate a certificate request 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 >> Certificates…