Managers of servers can set any message size limit they want.  Also
many usenet clients have a maximum size setting.

A few points to consider -
 the message has to be stored in its entirety on every single one of
the many thousands of news servers - this consumes disk space and
bandwidth worth perhaps hundreds, if not thousands of dollars.
 the longer the message, the longer it takes to download it - rural
dialups often have no more than 3 kilobits per second of usable
bandwidth ... and the longer it takes to find out whether the message
is even worth downloading.
 it is much more satisfactory from almost every point of view if
large messages are made into web pages, and a link with a short
description of the message is posted instead of the entire message -
you have several megabytes of web space available in your fidnet
account if you haven't already filled it up (web pages can be plain
text instead of HTML if the filename extension is ".txt").

If you do try posting the entire thing, *please* add a size note to
the subject.

No limits. But if it is too long many may not read it--human
nature and the modern short attention span!

no limit that I'm aware of - though some individual ISPs may have
their own standards. I really don't think any would cut a 20k story.
OTH, if there or logical breaks in the story you might consider
posting several parts rather than one lump. I just popped over and
looked at a directory of some of my longer posts, and saw a couple of
my posts were longer bigger than 20kb.