var a = window.open("a.htm", "", "width:300");
alert("宽 = "+ a.document.body.clientWidth);
alert("高 = "+ a.document.body.clientHeight);
alert("宽 = "+ a.document.body.clientWidth);
alert("高 = "+ a.document.body.clientHeight);
<script>
alert(this.document.body.clientWidth);
alert(this.document.body.clientHeight);
</script>