JavaScript - open a new window from button click

Hi,
I can't work out how to open a new window 900px wide by 800px high when the button is clicked?


<input name="button" type="button" class="formbutton_admin" id="button" onClick="javascript:preview_template(document.getElementById('template_name').value,document.getElementById('template_style').value)" value="Preview Template">


<script type="text/javascript">

function preview_template(name,style){      
            var name = name;
             var style = style;
            self.location = "preview.php?template_name="+name+"&style="+style;
}

 function MM_openBrWindow(theURL,winName,features) {
  window.open(theURL,winName,features);
}

</script>

Your comments and feedback would be appreciated.