Below is command to get size of exchange mailbox for user test1.
Get-MailboxStatistics -Identity test1 |select TotalItemSize
You can also select display name in output
Get-MailboxStatistics -Identity journaldb06 |select displayname, TotalItemSize
You can also list size of all mailbox and displayname of the user with below command:
get-mailbox | Get-MailboxStatistics |select displayname, TotalItemSize