When you run Get-Mailboxstatistics, you will get an overview of the size of all mailboxes in your organization.
To sort the mailboxes on size, you can pipe the output of the cmdlet Get-MailboxStatistics to Sort-Object TotalItemSize -desc. You will notice that Exchange gives you the TotalItemSize in Bytes. But you can change that since TotalItemSize is a so-called ByteQuantified Size Date, meaning you can apply a method to it to set the value in Kb, Mb, or even Gb if you want to.
The following single line will give you your top 5 largest mailboxes, sorted by TotalItemSize, expressed in Mb, the column will be entitled Total Mailbox Size in Mb!
[PS] C:\Get-MailboxStatistics | Sort-Object TotalItemSize -desc | Select-Object -First 5 | ft displayname,@{Expression={$_.TotalItemSize.Value.ToMb()};label="Total Mailbox Size in Mb"}
DisplayName Total Mailbox Size in Mb
———– ————————
Administrator 500
User4 477
User5 368
User9 359
User2 52