private void method() {
    commandDialog.setPreferredSize(new Dimension(100,100));
    - - -
    - - -   //Components added to dialogPanel

    commandDialog.add(dialogPanel);

    // Tried this as well: commandDialog.setSize(40, 40);
    commandDialog.validate();
    commandDialog.setVisible(true);
}

commandDialog.setSize(new Dimension(100, 100));
commandDialog.setResizable(false);

commandDialog.pack();
    commandDialog.setVisible(true);