codice:
    var _height = 100;
    var _width = 100;

    var _xpos = (screen.height - _height) / 2;
    var _ypos = (screen.width - _width) / 2;

    window.open("url", "nome", "height=" + _height + ",width=" + _width + ",top=" + _ypos + ",left=" + _xpos + ",screenY=" + _ypos + ",screenX=" + _xpos;