ページの中央にウィンドウを開くJSをカスタマイズします.

//            JS
    function openWin(url,w,h){
        if(w==""||w==null){
            w = "620" ;
        }
        if(h==""||h==null){
            h =  "650";
        }
        var winWidth = w; //        
	    var winHeight = h; //        
	    var winLeft = Math.floor((window.screen.availWidth-winWidth) / 2);
	    var winTop = Math.floor((window.screen.availHeight-winHeight) / 3);
	    var setting = "height=" + winHeight + ",width=" + winWidth + ",left=" + winLeft + ",top=" + winTop + "," + " toolbar=no, menubar=no, scrollbars=yes, resizable=yes,location=no, status=yes";
	    window.open(url,"newwindow",setting);
    }