Problems with components on InternalFrame

Question

I am currently creating a GUI with internalFrame. This is how my code looks like:

import javax.swing.*;

public class GUI extends JFrame {

private JDesktopPane desktop;
private JMenuBar menuBar;
private JMenu startMenu;
private JMenuItem travelInfo;
private JButton updateData;

public GUI()
{
    desktop = new JDesktopPane();
    updateData = new JButton("Update");
    createFrame();
    setContentPane(desktop);
    setJMenuBar(buildMenu());
}

private JMenuBar buildMenu()
{
    menuBar = new JMenuBar();

    startMenu = new JMenu("Start");
    menuBar.add(startMenu);

    travelInfo = new JMenuItem("TravelInfo");
    startMenu.add(travelInfo);

    return menuBar;
}

protected void createFrame()
{
    NewFrame frame = new NewFrame();
    frame.add(updateData);
    frame.setVisible(true);
    desktop.add(frame);
    }
}

And then my other class:

import javax.swing.JInternalFrame;

class NewFrame extends JInternalFrame {

    public NewFrame() {
        setSize(800, 600);
    }
}

And my main:

import javax.swing.JFrame;

public class Main {

public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI();
        }
    });
}

private static void createAndShowGUI()
{
    GUI frame = new GUI();
    frame.setSize(800, 600);
    frame.setResizable(false);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
}
}

What I want to do is to make the internalFrame that I create unsizeable so that it will always be as big as the JDesktopPane which is unsizeable. I am then going to add different menu option so that I can switch between the different frames so it kinda looks like a website. I am also wondering how I change the size of my button inside my internalFrame. I tried setting the frame.setResizable(false); for the internalFrame but it didn't work. Setting a size on the button didn't work out either... Somebody got an idea on how to solve this?