Below Powershell command can be used to export the list of mailboxes properties like DisplayName, totalItems,ItemCount for the list of users given in a text file to a export to a csv file
Get-Content “C:\name.txt” | Get-Mailbox | Get-MailboxStatistics | Select DisplayName,TotalItemSize,ItemCount| Export-Csv c:\MailboxStatistics.csv
Get-Content “C:\name.txt” | Get-Mailbox | Get-MailboxStatistics | Select DisplayName,{$_.TotalItemSize.Value.ToMB()},ItemCount| Export-Csv c:\MailboxStatistics.csv
Get-Content “C:\name.txt” | Get-Mailbox | Get-MailboxStatistics | Select DisplayName,{$_.TotalItemSize/1.0MB},ItemCount| Export-Csv c:\MailboxStatistics.csv
Get-Content “C:\name.txt” | Get-Mailbox | Get-MailboxStatistics | Select DisplayName,{$_.TotalItemSize.Value.ToGB()},ItemCount| Export-Csv c:\MailboxStatistics.csv