>a size limit of maybe 600 lines or so (Sizelim= 600) on the messages

As a matter of fact I'd make this more like 250 and tune it as necessary.
It depends entirely on the usual size of messages posted to the list (and
be sure you understand that Sizelim= counts the Received: headers as
message lines when messages are evaluated).  For some lists 250 may be
too small, of course.

Nathan