WSN is highly scalable, but millions of long posts on a very popular website can slow down database queries on that table severely and increase hardware requirements. This mainly applies to discussion forums (WSN Forum), of course. Post searches end up taking interminably long and putting pressure on the server.
One solution is to archive old posts in threads which haven't had a reply in over a year. First, make sure you're running WSN 8.0.0 Alpha 6 or later. Then, go into phpmyadmin (in your web hosting control panel) and click on the yourprefix_posts table. Go to the operations tab, and use that to copy the table structure and data to yourprefix_archivedposts.
Then run this SQL query at Admin -> Miscellaneous -> Advanced Options:
UPDATE {PREFIX}links SET archived=1, threadclosed=1 WHERE lastpost < UNIX_TIMESTAMP()-86400*365;
You can change 365 to a desired number of days.
Finally, you'll need to run a utility to delete posts from the regular posts table. Save the following as a 1postdeleter.php file in your WSN folder:
require 'start.php';
if (!$start) $start = 0;
$perpage = 50;
$q = $db->select('id', 'linkstable', 'archived=1', "ORDER BY id ASC", "LIMIT $start,$perpage");