If you have access to the mailbox, both Outlook and OWA will readily show you this info. In Outlook, right-click on the root folder > Data file properties > Folder size > Server data. In OWA, go to Settings > General > Storage (or directly https://outlook.office.com/mail/options/general/storage)
As an admin, you can also get the same data via PowerShell:
Get-MailboxFolderStatistics ******@domain.com | select Name,Folder*Size,Items*