Open Exchange Powershell :
Get-MailboxStatistics -Database "Mailbox Database" | Select-object PrimarySmtpAddress, DisplayName, ItemCount, TotalItemSize | Sort-Object TotalItemSize -Descending | Export-CSV C:\MBSizes.csv
It`s simple when you know how ! Microsoft Exchange 2010,Microsoft Exchange 2013,Microsoft Exchange 2016 How to Export Mailbox size exchange server