Exchange 2013 Statistics Mailbox Capacity-powershell

Hello everyone, today get a demand, is to count all user mailbox capacity, for the OFFICE365 migration to prepare, because we buy on the O365 capacity is limited, their own management did not limit, some users have used dozens of G.
This requirement I do not know how to use the graphical interface to do, check on the Internet, found that some of the documents are written in a different way than the actual. is actually a PowerShell statement to implement.
Website link is:
https://technet.microsoft.com/en-us/library/bb124612 (v=exchg.160). aspx

I would have thought that if this command supported-filter, I could have an OU with an OUs until I found the following sentence:
The Get-mailboxstatistics cmdlet requires at least one of the following parameters to complete Successfully:server, Datab ASE, or Identity.

Is that you must specify a server, or database or ID

So, I have the whole sentence:
Get-mailboxstatistics-server Mail server name | where {$. Objectclass-eq "Mailbox"} | Sort-object totalitemsize-descending | FT @{label= "User"; expression={$. Displayname}},@{label= "TotalSize"; expression={$_. Totalitemsize}}
| Export-csv A.csv

In fact, there are many ways, but this is relatively simple. Also, you can specify the field with Select-object, now the size result is much stronger than before, directly to you to do the conversion, or you have to construct an expression to calculate.

To ensure that you have write permission to the current directory, you cannot save the file.

Exchange 2013 Statistics Mailbox Capacity-powershell