Exchange Online has a number of PowerShell commands that will give you detailed level access into a single or group of mailboxes. One of the more powerful commands is Get-MailboxStatistics.
To get more information on the command, please use this Microsoft site.
Connect to Exchange Online with PowerShell
To view Mailbox Statistics for a single mailbox
- Get-MailboxStatistics -Identity <UserIdentity>
To view a formatted list of all the parameters on a single mailbox
- Get-MailboxStatistics -Identity <UserIdentity> | Format-List
To view Mailbox Statistics for all mailboxes
- Get-Mailbox | Get-MailboxStatistics