I have a CardLayout within which I have panels constituting each 'card'. However, the panels refuse, seemingly, to respond to my sizing requirements.

doublePane = new JSplitPane(JSplitPane.VERTICAL_SPLIT, comboPanel, lowerPanel);

comboPanel.setSize(new Dimension(100, 600)); // totally ineffective
lowerPanel.setSize(new Dimension(1100, 600));

JPanel displayPanel = new JPanel();
displayPanel.add(doublePane);
displayPanel.setSize(1200, 600);

mainPanel.setLayout(new CardLayout());
mainPanel.add("Main", displayPanel);

Any ideas would be appreciated...

You cannot control the size of controls (including panels). The setSize() method should never be relied upon.