var test_format:TextFormat = new TextFormat();
test_cb.setStyle("textFormat", test_format);

test_cb.dropdown.rowHeight = 40;

test_cb.dropdown.setStyle("cellRenderer", CustomCellRenderer);

package {
    import fl.controls.listClasses.CellRenderer;
    import flash.text.TextFormat;
    public class CustomCellRenderer extends CellRenderer {
        public function CustomCellRenderer() {
            var format:TextFormat = new TextFormat(null, 24);
            setStyle("textFormat", format);
        }
    }
}