This is not possible at the moment and  it is not simple at all. The list
owner decides  the frequency  and maximum size  of the  digests. LISTSERV
maintains a single  digest file, which does not require  that the list be
archived, and when the limit is reached it just sends it out. In order to
do  what you  want, one  would first  have to  require that  the list  be
archived. Then for  each user a customized digest would  have to be built
after the fact from the archive, where each digest could have a different
size, topic list, etc. This in  turn means DISTRIBUTE cannot be used, and
the  machine  +  network  cost  is much  higher.  Finally  you  have  the
interesting  issue of  when the  digest  is sent.  In fact,  most of  the
complaints  are not  about the  frequency, but  about the  time when  the
digest is issued. For daily digests this  is always at the wrong time for
someone's time zone,  and for weekly digests you have  people who want it
on Friday evening, people who want it on Sunday morning, etc.
 
  Eric