A good layout  manager for variable sized forms is GridBagLayout.

Sorry I cant answer your previous question, but perhaps use JFrame.

Be warned though ... GridBagLayout is insanly obscure, and painfull to
use at first.

A regular GridLayout would be very easy to use for a calculator.

For a simple calculator , GridLayout is already sufficient though...

Never tried, but i think you can use a window with a desktoppane and a
JInternalFrame.
Whith Jinternalframe you can have those buttons  disabled.

For a swing container without maximize and minimize  buttons extend a
JDialog, try something like this:

class TestDialog extends javax.swing.JDialog
{

TestDialog()
{
setDefaultCloseOperation(javax.swing.JDialog.DO_NOTHING_ON_CLOSE);
addWindowListener( new java.awt.event.WindowAdapter()
{
public void windowClosing(java.awt.event.WindowEvent e)
{
System.exit(0);
}
} );
setResizable(false);
setBounds(0,0,100,100);
setVisible(true);
}

public static void main(String[] args)
{
TestDialog thisDialog = new TestDialog();
}

}