csseur3 you can take advantage of the function forum_last_topic_id() to fetch any info you'd like about the last topic in a forum.
Here is an example of how to use it:
topic_title(forum_last_topic_id());
That will echo the last topic title to the screen.
Or to store it and manipulate it further:
$last_topic_title=get_topic_title(forum_last_topic_id());
(you'll need to do some PHP to limit the length, like the substr function, see php.net)
$last_topic_title=substr($last_topic_title,0,40);
or you can fetch everything about the last topic:
$last_topic=get_topic(forum_last_topic_id());
and then you can determine things like how many posts it has:
echo "Most recent topic has $last_topic->topic_posts posts";