This is how I included div on Master page(outside ContentPlaceHolder):
<div id="DivTest" style="width: 200px;">
</div>
This is the code I wrote on child page(inside ContentPlaceHolder):
<script language="javascript" type="text/javascript">
var content_div = document.getElementById('DivTest');
var content_width = content_div.clientWidth;
alert('content_width=' + content_width + 'px');
</script>
It worked.