width

значения свойства:

auto
px
%
inherit

css/style.css

.a {
width: 300px;
height: 150px;
background-color: #3BA9DE;
}
#div1 {
height: 150px;
background-color: #56A31D;
}
.b {
height: 150px;
background-color: #852E76;
}

.a {
 width: 300px;
 height: 150px;
 background-color: #3BA9DE;
}

#div1 {
 height: 150px;
 background-color: #56A31D;
}

.b {
 height: 150px;
 background-color: #852E76;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="a"></div>
<br />
<br />
<div id="div1"></div>
<br />
<br />
<div class="b"></div>
<script>
/*JavaScript*/
document.getElementById("div1").style.width = "300px";
/*jQuery*/
$(document).ready(function () {
$(".b").css("width", "300px");
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="a"></div>
<br />
<br />
<div id="div1"></div>
<br />
<br />
<div class="b"></div>
<script>
/*JavaScript*/
document.getElementById("div1").style.width = "300px";
/*jQuery*/
$(document).ready(function () {
$(".b").css("width", "300px");
});
</script>
</body>
</html>