Unfortunately, you can't adjust the message header fonts or the fonts in the configuration screens.  These fonts are fixed in the code.  The workarounds are to either use a screen magnifier or decrease the screen resolution (changing system fonts will not help).

FWIW, I run by 1920x1080 laptop at 1600x900.

The font size in the sending mail dialog box, where you enter the recipients email address, is so small I can't read it. The font size in the body of the email is perfect, and I can see where I can adjust that. But where can I adjust or make larger the font size in the sending recipient, subject, cc dialog boxes?