.mynode {
width:120px;
padding:50px;
border: 1px solid black;
font-size: 20px !important;
}
When you draw a chart, use this line:
// Create the chart.
var chart = new google.visualization.OrgChart(document.getElementById('chart_div'));
// Draw the chart, setting the allowHtml option to true for the tooltips.
chart.draw(data, {'allowHtml':true, 'allowCollapse':true, 'nodeClass': 'mynode'} );
You will lose most of the default css using this method.