In the previous two Exchange Server editions viewing a user mailbox size was a task that could only be done via the Exchange PowerShell console.
Exchange 2013 changes that and makes this task very simple and now you can view the size via the GUI interface.
To view a user mailbox size Log in to EAC (https://exchangeserver/ecp)
In the recipient -> Mailbox menu double click on the user mailbox
Click on Mailbox Usage and view the current size and limit