<div class="bs-example">
    <div class="panel-group" id="accordion">
        <?php if($_product->getSizeDescription()): ?>
          <div class="panel panel-default">
              <div class="panel-heading">
                  <h4 class="panel-title">
                      <a data-toggle="collapse" data-parent="#accordion" href="#collapseTwo" class="acoll collapsed"><span class="glyphicon glyphicon-plus"></span> Størrelse & Pasform</a>
                  </h4>
              </div>
              <div id="collapseTwo" class="panel-collapse collapse">
                  <div class="panel-body">
                      <?php echo $_product->getData('size_description'); ?>
                  </div>
              </div>
          </div>
        <?php endif; ?>
    </div>
</div>

<script type="text/javascript">
     jQuery(document).ready(function(){
      // Add minus icon for collapse element which is open by default
      jQuery(".collapse.in").each(function(){
          jQuery(this).siblings(".panel-heading").find(".glyphicon").addClass("glyphicon-minus").removeClass("glyphicon-plus");
      });
      // Toggle plus minus icon on show hide of collapse element
      jQuery(".collapse").on('show.bs.collapse', function(){
          jQuery(this).parent().find(".glyphicon").removeClass("glyphicon-plus").addClass("glyphicon-minus");
      }).on('hide.bs.collapse', function(){
          jQuery(this).parent().find(".glyphicon").removeClass("glyphicon-minus").addClass("glyphicon-plus");
      });
    });
</script>