function finestra2(w,h)

{
var l = Math.floor((screen.width-w)/2);
var t = Math.floor((screen.height-h)/2);


window.open("pagina.htm","","width=" + w + ",height=" + h + ",top=" + t + ",left=" + l);

}

la chiamata...
<a href="javascript:finestra(550,413);">