The cmdlet below will show you how to get a Mailbox Folder statistics for specific user, the cmdlet will also sort the folders by Folder size when the largest folder size will be on the top of the list.
Get-MailboxFolderStatistics -Identity USERID | select name, foldersize | Sort-Object foldersize -des
To export the date to a CSV file you the cmdlet with export-csv:
Get-MailboxFolderStatistics -Identity USERID | select name, foldersize | Sort-Object foldersize -des
| export-csv folderlist.txt