How to resize JPanel that uses GroupLayout

I have a program in which there are 3 combo boxes and their labels in a Group Layout.  When the window first appears, only the first combo box contains options, so the window stretches only large enough to fit the labels and combo boxes.  After a person selects an item from the first combo box, the 2nd one is filled with associated data.  This associated data is sometimes causes the combo box to be too wide and expands beyond the window.  I have been playing with the JPanel's setPreferredSize and setMaxSize, but nothing appears to be happening.  Does anyone know how I'm supposed to change the width of the panel so that the drop-downs fit properly.

Here is the code I am currently trying to work on:

projectListener = function(){

var Project_Select = Project_Select_Field.getSelectedItem();

var proj;

var projSel;

var panelWidth = myPanel.getWidth();

var panelHeight = myPanel.getHeight();

var comboWidth = Project_Select_Field.getPreferredSize().width;

//if-statement to determine which list to use to populate.

newWidth = Project_Select_Field.getPreferredSize().width;

var diffWidth = 0;

if (newWidth > comboWidth) {

   diffWidth = newWidth - cojoWidthbb

}

myPanel.setPreferredSize((panelWidth + diffWidth), panelHeight);

Does anyone know what I'm doing wrong?

Chris