You can change the font of the New Mail message list, the Folder list, and of the message reader window (will not affect HTML content).  When each window is active click the font icon located in the toolbar of the mail window (it is the blue, lower case "f").

As for the cause, I don't know.  The files that store this the folder layouts might have gotten corrupted and were rebuilt with default settings.  This is just conjecture though.  Also, there is a function that allows you to assign a default window layout to a folder window but this isn't something you could do accidentally.  I would not affect the message reader window anyway.

For some reason, all of a sudden, the text within the various windows in Pegasus has become smaller. The buttons and the "To" and "From" section seems normal, but the text within the windows is smaller. Did I do something inadvertently to cause that, and how do I get it bigger?

Some fonts are of fixed size and can be very small on a hi-res monitor (eg: To, Subject and CC fields as well as configuration windows to name a few).  This is what you might be experiencing if you changed monitors or increased the resolution.  Other fonts are configurable.  Provide some specific examples of which fields and which windows you see the font problem in and we can advise whether they are adjustable or not.

Thanks. It doesn't seem to happen in windows when I reply to an e-mail or send a new e-mail. However, in the original e-mail that enters the New Mail folder it happens. What is in the New Mail Folder itself also now has the smaller font. It also has happened in the Folders window and any e-mails I open from there.

 I have not changed monitors and this just started happening yesterday - seemingly out of the blue.

 Thanks