public class Test3 extends JFrame{
    private JTextField textField;
    public Test3() {
        getContentPane().setLayout(new FlowLayout(FlowLayout.CENTER, 5, 5));
        
        JComboBox comboBox = new JComboBox(new String[]{"1","2","3"});
//        comboBox.setSize(new Dimension(60,10));//doesn't work
        getContentPane().add(comboBox);
        
        textField = new JTextField();
        textField.setSize(100,50);//do not work either
        getContentPane().add(textField);
        textField.setColumns(10);
        this.pack();
        this.setVisible(true);
    }
    public static void main(String...args){
        new Test3();
    }
}