https://o365reports.com/2020/10/21/export-office-365-mailbox-size-report-using-powershell/

As an admin, it is an important task to periodically monitor Office 365 mailbox size and quotas. Else, it will end up in error like ‘mailbox size limit exceeded,’ ‘unable to send/receive messages due to mailbox quota,’ etc.

So, how will you check the mailbox size in Office 365? You can use either the Exchange admin center or PowerShell to get a mailbox usage report. But the Exchange admin center can show limited information only. With PowerShell, you can get a detailed mailbox size report. However, you need to combine multiple cmdlets like Get-Mailbox, Get-MailboxStatistics, etc.

To ease your work, we have created a handy PowerShell script to export mailbox size and quotas to a nicely formatted CSV file.

Download Script: GetMailboxSizeReport.ps1

Script Highlights:

• The script uses modern authentication to connect to Exchange Online.
• The script can be executed with MFA enabled account too.
• Exports report results to CSV.
• You can choose to either “export mailbox size of all mailboxes” or pass an input file to**“get usage statistics of specific mailboxes”** alone.
• Allows to use filter to get user mailboxes’ size alone
• Allows to use filter to get shared mailboxes’size alone.
• Automatically installs the EXO (Exchange Online) V2 module (if not installed already) upon your confirmation.
• The script is scheduler friendly. I.e., Credential can be passed as a parameter instead of saving inside the script.

Office 365 Mailbox Size Report – Sample Output:

The exported mailbox size usage report looks like screenshot below:

The exported mailbox size and quota report contains following attributes: