Office365: What Is The Size Of Users Mailbox?

Get-Mailbox | Get-MailboxStatistics | Select-Object DisplayName, @{name="TotalItemSize (GB)";expression={[math]::Round((($_.TotalItemSize.Value.ToString()).Split("(")[1].Split(" ")[0].Replace(",","")/1GB),2)}},ItemCount | Sort "TotalItemSize (GB)" -Descending

If we need to export the results into CSV file:

Get-Mailbox | Get-MailboxStatistics | Select-Object DisplayName, @{name="TotalItemSize (GB)";expression={[math]::Round((($_.TotalItemSize.Value.ToString()).Split("(")[1].Split(" ")[0].Replace(",","")/1GB),2)}},ItemCount | Sort "TotalItemSize (GB)" -Descending | Export-CSV C:\TMP\Office365-MailboxSize-Report.csv