Растянуть <div> через заданную width

$(function() { 
  var e = $('.two'), p0 = $('.one'), p1 = $('body'); 
  var w = (e.outerWidth() / p0.outerWidth()) * p1.outerWidth(); 
  e.css({width: w}); 
});

.one {width:300px;height:50px;border:1px solid black} 
.two {width:100%;height:50px;background-color:red}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
 
<div class="one"> 
  <div class="two"> 
  </div> 
</div>