dovecot - May 2008 - how add size and vsize filed to a maildir filename ?

Hi to all,
I'm a sys admin in a college, and we're using Dovecot as IMAP/POP3 
server and delivery; we're also patch dovecot to add managesieve 
capability. I've a question:

I've read on dovecot's wiki, that is possible improve the performance on
maildir file by adding ,S=<size>,W=<vsize> fields on filename. How
is
possible to do that ? I've search on all the wiki and on the mailing 
list, but I didn't find nothing about modify the maildir file name.

Someone can help me ?

Best regards,
    Marco