Export a list of all mailboxes in Exchange

Get-MailboxDatabase | Get-MailboxStatistics | Export-CSV c:\temp\mailboxdata.csv

or

You can then be more specific about the data you want to output using any of the fields from the first command and put it in an order:

Get-MailboxStatistics -Database "Mailbox DB01" | Select DisplayName, ItemCount, TotalItemSize | Sort-Object TotalItemSize -Descending | Export-CSV c:\temp\mailboxdata.csv

or
You can just leave out the database parameter if you want information on all mailbox databases:

Get-MailboxDatabase | Get-MailboxStatistics | Select DisplayName, ItemCount, TotalItemSize | Sort-Object TotalItemSize -Descending | Export-CSV c:\temp\mailboxdata.csv