Comment je fixe le format et la taille d'un champs

import java.awt.Color;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
 
import javax.swing.JFrame;
import javax.swing.JTextField;
 
public class Test
{    
    public static void main(String args[])
    {  
        final JTextField txt = new JTextField();
        
        txt.addKeyListener(new KeyListener()
        {
            public void keyReleased(KeyEvent e) 
            {
                if (txt.getText().matches("^[A-Z]{2}[0-9]+$"))
                    txt.setForeground(Color.BLACK);
                else
                    txt.setForeground(Color.RED);
            }
            
            public void keyTyped(KeyEvent e) {}
            public void keyPressed(KeyEvent e)  {}
        });
        
        JFrame frame = new JFrame();
        frame.add(txt);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}