Is there a reason you want to use the theme_preprocess_views_view_fields hook?
Why not add the attribute Content: type to your View fields (see image), and then check the content type within a View template:
Then your code in views-view-fields--[my_view]--[my_display].tpl.php could look something like this:
<?php
//check content type
if($fields['content_type']->content == 'My content type') {
//trim description by 200 chars (respecting full words)
$pos = strpos($fields['field_my_description]->content, ' ', 200);
substr($fields['field_my_description]->content, 0, $pos);
}
?>
<h3><?php print $fields['title']->content; ?></h3>
<div class="my-description">
<?php print $fields['field_my_description']->content; ?>
</div>
I haven't tested or tried the above code, but the proof of concept should be enough to help you along.
Using a Views template instead of a hook is also easier for site builders / themers to debug or change.