Well, fat32 knows of files 4GB or smaller, so that's the limmit.  I keep my 
saved messages, and every new year's day I rename the mailbox, and let OE 
create a new in box.  You could do the same thing with in boxes, using move 
to folder to select a year at a time.  ZI think though that 4GB of messages 
will take a while to collect.