When set 'fit' property to true, the layout will auto full fill its parent container. Try this code:
<body>
<div class="easyui-layout" fit="true">
<div data-options="region:'north',border:false" style="height:60px;"></div>
<div data-options="region:'west',split:true,title:'West'" style="width:150px;"></div>
<div data-options="region:'south',border:false" style="height:50px;"></div>
<div data-options="region:'center',title:'Center'"></div>
</div>
</body>
or
<body class="easyui-layout">
<div data-options="region:'north',border:false" style="height:60px;"></div>
<div data-options="region:'west',split:true,title:'West'" style="width:150px;"></div>
<div data-options="region:'south',border:false" style="height:50px;"></div>
<div data-options="region:'center',title:'Center'"></div>
</body>