I've never tried to do quite what you are attempting, so I don't know
how helpful this will be.  If you post some example code, it may make
it easier for people to give you help.

The JOptionPane usually sizes itself to fit what it contains. As an
experiment, create a JTextArea using new JTextArea(), and add it to a
JScrollPane with new JScrollPane(the JTextArea).  When you layout the
JScrollPane and show it on the screen, you won't be able to see the
scroll pane because it is 1 by 1 pixel in size.  Now go back and
create the JTextArea with new JTextArea(80, 80).  Now you can see the
scroll pane because the text area has a size that the viewport can use
to set its size.

I'm not sure what will happen if you remove and then add different
objects to the scroll pane.  You might try using a card layout, and
having a JTextArea or something else with a well defined size as the
first object in the layout.  You could then add and remove the other
members to the card layout and only show the object you want to show.
I would guess that you will have to play around with setting
preferred, or perhaps minimum and maximum sizes, to get the exact
effect you want.

Good luck,

John

Layouts are one of the most difficult things to learn in Java.  All
layout managers respond to different conditions, some
minimum/maximum/preferred size, some the size of the container and some
both.

Remember that if you add a component to a ScrollPane constructor that
component is the ViewPort and will follow the rules of the
ScrollPaneLayout.  It appears to be very similar the the center of a
BorderLayout in action.

I wrote a little test program that shows the size of a JPanel in a
JScrollPane.  The JScrollPane only shows vertical scroll bars, never
horizontal.  The JScrollPaneLayout will allow the JPanel to expand
larger than its preferred or maximum size (just as BorderLayout would).
It can never be smaller than the minimum width but if the JScrollPane
has been resized smaller than that minimum size some of the JPanel is
not visible.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class test5 extends JPanel {
    public test5() {
    }

    public void paintComponent(Graphics g) {
        g.setColor(getBackground());
        g.fillRect(0,0,getWidth(),getHeight());
        g.setColor(Color.WHITE);
        g.drawString("Width:   " + Integer.toString(getWidth()),10,20);
        g.drawString("Height:  " + Integer.toString(getHeight()),10,40);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame f = new JFrame();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                test5 p = new test5();
                p.setBackground(Color.RED);
                p.setMinimumSize(new Dimension(100,75));
                p.setPreferredSize(new Dimension(200,150));
                p.setMaximumSize(new Dimension(400,300));

                JScrollPane sp = new JScrollPane(p,
                 JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED,
                 JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

                f.add(sp);

                f.pack();
                f.setVisible(true);
            }
        });
    }
}

If you want to get more specific about how you want your layout to look
and what components are on your JPanel, we can probably get you closer
to your goal.