Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | ?{$_.TotalItemSize -gt 2GB} | Select DisplayName,TotalItemSize
Above command will only get you the mailboxes that are greater than 2GB.
In case if you want query per database then you can use:
Get-MailboxStatistics -Database DBName | ?{$_.TotalItemSize -gt 2GB} | Select DisplayName,TotalItemSize
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