I have a client whose site has products with some very long product names which can break various places when on a fixed width site. One such place is on the tell a friend page.

No amount of adjustment of the CSS operating on the <legend> tag had any effect whatsoever on reducing the width, and the fix I found was to replace the <legend>tags in tpl_tell_a_friend_default.php with heading tags

Had a similar issue with long category names breaking the layout on the Advanced Search page; here it was necessary to reduce the displayed width of the categories dropdown and this was achieved by adding an id to tpl_advanced_search_default.php ... at line 34, find
Code:
 <div class="floatLeft"><?php echo zen_draw_pull_down_menu('categories_id', zen_get_categories(array(array('id' => '', 'text' => TEXT_ALL_CATEGORIES)),0 ,'', '1')); ?></div>
and replace with
Code:
 <div class="floatLeft"><?php echo zen_draw_pull_down_menu('categories_id', zen_get_categories(array(array('id' => '', 'text' => TEXT_ALL_CATEGORIES)),0 ,'', '1'), '', 'id="myPullDown"'); ?></div>
Then set the width of #myPullDown in your stylesheet.