This is one of those things that you may not use often but you are almost guaranteed to need at some point. Below is an easy powershell script/command for you to use which will export a list of all your mailboxes and their sizes, into a CSV file.

Get-MailboxStatistics | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label=”TotalItemSize(MB)”;expression={$_.TotalItemSize.Value.ToMB()}},ItemCount > MailboxSizes.CSV

This is sorted by largest mailbox first and includes the name of the mailbox, the size of the mailbox (in MB) and the number of items in each mailbox.