Pane resizing has stuck between sessions in my test.  If you running the main Pegasus Mail window maximized try windowing it stretched full screen and see if the new sizing will stick.  Check the "Save the Pegasus Mail desktop state between sessions" option as well.  The help file doesn't reference pane size it's worth consideration during troubleshooting.

How can I change the size of the preview area. I would like more of the email to be visible and fewer entries in the message list.

Locate the divider bar between the panes and drag it resize the panes (there is a vertical one too).  The divider bars are barely visible but slowly dragging the cursor from pane to pane will identify them.

I assume your are talking about Preview mode.

Thanks Brian, I know about that but what is happening is that even though I position it the way I want, the next time I open Pegasus it is the same as before my change. What I want is to make the new position the default.