bootstrap搜索框自动适应缩放比例

<nav class="navbar navbar-default" role="navigation">
   <div class="navbar-header">
       <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
           <span class="sr-only"></span>
           <span class="icon-bar"></span>
           <span class="icon-bar"></span>
           <span class="icon-bar"></span>
       </button>
   </div>
   <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
       <ul class="nav navbar-nav">
           <li class="dropdown">
               <a href="Area.html">成都市 <b class="glyphicon glyphicon-menu-down"></b></a>
           </li>
       </ul>
       <form class="navbar-form navbar-left" role="search-box">
           <input type="text" class="form-control"  onclick="javascript:window.location.href='Search.html';"  placeholder="职位搜索">
       </form>
       <ul class="nav navbar-nav navbar-right">
           <li class="dropdown">
               <a href="#" class="dropdown-toggle" data-toggle="dropdown">筛选<b class="glyphicon glyphicon-list"></b></a>
           </li>
       </ul>
       <div class="clearfix"></div>
   </div>
</nav>