Роман Андреевич,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
html *{padding:0;margin:0}
li{border:3px solid #0000FF;float:left;box-sizing:border-box; height: 120px;}
ul{list-style:none}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
function slider_size() {
var ul = $("ul"),
widthAll = 0;
$("li", ul).each(function(indx, element) {
this.style.width = document.documentElement.clientWidth + "px";
widthAll += this.offsetWidth
});
ul.width(widthAll)
}
$(window).resize(slider_size).trigger("resize")
});
</script>
</head>
<body>
<section>
<nav>
<ul>
<li>
//.....какой то контент
</li>
<li>
//.....какой то контент
</li>
<li>
//.....какой то контент
</li>
<li>
//.....какой то контент
</li>
</ul>
</nav>
</section>
</body>
</html>