<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<!--<meta name="viewport" content="width=device-width,initaial-scale=1,maximum-scale=1,minimum-scale=1,user-scalable=no" />-->
<title>Bootstrap</title>
<link rel="stylesheet" type="text/css" href="css/css/bootstrap.min.css"/>
<script type="text/javascript" src="js/jquery-3.2.1.min.js"></script>
<script type="text/javascript" src="js/js/bootstrap.js"></script>
<style type="text/css">
.box{
/*width: 300px;*/
height: 200px;
background: red;
}
/*@media screen and (min-width: 100px) and (max-width: 640px) {
.box{
width: 100%;
height: 200px;
background-color: yellowgreen;
}*/
}
</style>
</head>
<body>
<div class="box col-lg-3 col-md-4 col-sm-6 col-xs-12"></div>
可以了
你没有写css样式控制大小