Powershell - get total size of maliboxes from a particular AD Organizational Unit?

I am looking to run a command within Exchange Powershell that allows me to gather only the total size of all mailboxes combined, for all mailboxes that live in an OU. The OU also has nested OU's as well so I'd need to include Mailboxes recursively within this OU.

Here's the command I got so far, I'm just not sure if it's getting nested mailboxes within the OU, and I'm not sure how to not report out each individual mailbox, but instead just calculate the total of all of them together?    

Also this is Exchange 2007 but some mailboxes within the OU live in Exchange 2010, but I think this reports accurately on those as well.

Get-Mailbox | where {$_.DistinguishedName -like "*OU=IT,OU=Business,OU=Cafe,DC=cafenet,DC=com*"} | Get-MailboxStatistics | Add-Member -MemberType ScriptProperty -Name TotalItemSizeinMB -Value {$this.totalitemsize.value.ToMB()} -PassThru | Format-Table DisplayName,TotalItem*

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Or if there's an easier way within the Exchange Management Console that could work as well.
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Minecraft_ EndermanCommented:
To get the users in a particular AD Organizational Unit, we can use the –OrganizationalUnit and -DomainController parameter.

Get-Mailbox -OrganizationalUnit <OrganizationalUnitIdParameter> -DomainController <Fqdn> | Get-MailboxStatistics | Select-Object DisplayName, TotalItemSize | export-csv C:\report.csv

Refer from:
https://technet.microsoft.com/zh-cn/library/bb676556(v=exchg.80).aspx (2007)
https://technet.microsoft.com/zh-cn/library/bb123685%28v=exchg.141%29.aspx?f=255&MSPPError=-2147217396 (2010)
garryshapeAuthor Commented:
Here's what I did instead, which makes it easier:

get-mailbox -OrganizationalUnit "OU=IT,OU=Business,OU=Cafe,DC=cafenet,DC=edu" | Get-MailboxStatistics | Select DisplayName,@{label='SizeInMB';expression={$_.TotalItemSize.Value.ToMB()}  }

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Then if I want to simply sum up the total of the SizeInMB column, I just add to the end of the line:

| Measure-Object 'SizeInMB' -Sum

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