Wouldn’t it be good to have an easy way to get the total size of all mailboxes you host in Exchange Online? Yes?
Challenge accepted! Using the new Exchange Online PowerShell module V2, it is a simple 1-liner. The following command will give you the sum in Megabytes:
((get-exomailbox -ResultSize Unlimited | get-exomailboxstatistics).TotalItemSize.Value.ToMB() | measure-object -sum).sum