var w=400, h=600, pos=getCenterPos(w, h);
var opts = 'width='+w+', height='+h+', left='+pos.left+', top='+pos.top+', menubar=no, status=no, boolbar=no, location=no';
window.open('url here', 'title here', opts);

function getCenterPos(w, h) {
  var dualScreenLeft = !window.screenLeft ? window.screenLeft/*Most browsers*/ : window.screenX/*Firefox*/
  var dualScreenTop = !window.screenTop ? window.screenTop : window.screenY;

  var left = window.screen.width - w;
  left = left > 0 ? left/2 : 0;

  var top = window.screen.height - h;
  top = top > 0 ? top/2 : 0;

  return {
    left: !dualScreenLeft ? left : left + dualScreenLeft,
    top: !dualScreenTop ? top : top + dualScreenTop
  };
}