There doesn't seem to be an easy way to just see how much space all your mailboxes are using in the Microsoft 365 Exchange Online portal. Additionally, while the output from Get-MailboxStatistics has a value "TotalItemSize" it's a string and not a raw value, so it requires a little parsing to w0rk. Instead of just giving you the number of bytes (or MB or GB) it gives you a string like this: 38.25 GB (41,065,726,825 bytes)
Well, with a little regex and the Measure-Object cmdlet, we can get to where we want.
(Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics).TotalItemSize.Value -replace '.*?\(|[^\d]*','' | Measure-Object -Sum
It will take a little while, but in the end you'll get the total. :-)
Count : 486
Average :
Sum : 2339040542238
Maximum :
Minimum :
Property :
That's 2.12TB of data in 486 mailboxes, in this case.