The following one liner will export all mailboxes with size details to a csv file. Save it at as a .ps1 script for ease of use.

Get-MailboxStatistics | where {$_.ObjectClass -eq “Mailbox”} | Sort-Object TotalItemSize -Descending | select-object @{expression={$_.DisplayName}},@{expression={$_.TotalItemSize.Value.ToMB()}},@{expression={$_.ItemCount}},@{expression={$_.StorageLimitStatus}} | Export-Csv mailboxes.csv