Get-MailboxStatistics cmdlet gives TotalItemSize which is the mailbox size. But the TotalItemSize doesn’t contain the mailbox size in numbers, it’s a PowerShell deserialized object.

This command displays the mailbox size from the value property:

(Get-MailboxStatistics -Identity username).TotalItemSize.Value

Example output: 10.43 GB (11,202,063,583 bytes)

But how to make it usable in creating reporting or other purpose. Convert that value to a string, split at ‘(‘ and take the first item in the split array.  Here is the command that gives you usable mailbox size.

(Get-MailboxStatistics -Identity username).TotalItemSize.Value.ToString().Split(“(“)[0]

Example output: 10.43 GB

If you want in all in bytes, use this cmdlet:

(Get-MailboxStatistics -Identity username).TotalItemSize.Value.ToString().Split(“(“)[1].Split(” “)[0].Replace(“,”,””)

Example output: 11202063583

Hope it is helpful for you.