[Dovecot] how add size (, S=size) and virtual size (, W=vsize) fields 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 files 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