$(function(){
          var $comment = $('#comment');
          $('.bigger').click(function(){
              if(!$comment.is(":animated")){
                  if($comment.height() < 500){
                      //$comment.height($comment.height() + 50);
                      $comment.animate({height:"+=50"},400);
                  }
              }
          });
           $('.smaller').click(function(){
               if(!$comment.is(":animated")){
                   if($comment.height() > 50){
                       //$comment.height($comment.height() - 50);
                       $comment.animate({height:"-=50"},400);
                   }
               }
           });
           $('.up').click(function(){
              if(!$comment.is(":animated")){
                  $comment.animate({scrollTop:"-=50"},400);
              }
           });
           $('.down').click(function(){
               if(!$comment.is(":animated")){
                   $comment.animate({scrollTop:"+=50"},400);
               }
           });
       }); 

<div class="msg">
       <div class="msg_caption">
           <span class="bigger">放大</span>
           <span class="smaller">縮小</span>
           <span class="up">向上</span>
           <span class="down">向下</span>
       </div>
        <div>
            <textarea id="comment" rows="8" cols="20">
                 水電費水電費爽膚水法薩芬沙發放松放松沙發
                當爽膚水飛沙發發沙發發發發
                水電費水電費爽膚水法薩芬沙發放松放松沙發
                當爽膚水飛沙發發沙發發發發
            </textarea>
        </div>
    </div>

.bigger , .down , .up , .smaller{
    background-color: #f1a55c;
}