There are various reasons for knowing a Users mailbox size. For my purposes, I needed to plan a Migration from one Office365 Tenant to a New Office365 Tenant. Instead of purchasing a Tool to do the Migration, we opted to simply duplicate the licenses and do a Drop and Drag. When looking at the Mailbox size we do have to remember there are two mailboxes that we must check per user. The actual mailbox and Archive Mailbox.
Start off connect to Office365 Exchange Online. Once connected there are two primary cmd-lets we are going to use.
First is the Get-mailbox this will get you a list of all mailboxes on the connected Exchange, Note this will include active and inactive mailboxes.
The second is Get-mailboxstatistics this will return information specific to the mail requested.
Get Mailbox Size
The command then reads as follows:
Get-Mailbox | Get-MailboxStatistics | Select displayname, totalitemsize To Export simply add the Export tags and set a location where to drop the CSV file.
Get-Mailbox | Get-MailboxStatistics | Select displayname, totalitemsize | Export-Csv C:\Temp\CompanyMaiboxSize.CSV -NoTypeInformationGet Archive Size
The Command reads very similarly to getting the Mailbox size with the mail difference bain the -Archive Tag, this will only return mailboxes with Archive Enabled attribute.
Get-Mailbox -Archive | Get-MailboxStatistics | Select displayname, totalitemsize | Export-Csv C:\Temp\CompanyMaiboxSize.CSV -NoTypeInformation