How do I change the default font size for reading the body of messages in Mail 14.0
Mail &gt; Preferences &gt; Fonts &amp; Colors provides selectors for font style, size, usage (list, text body), and color.
Here's the Fonts &amp; Colors preference pane:

To change the text font, style, and size, push the &quot;Select&quot; button that appears at the right end of the line &quot;Message font.&quot; Doing so brings up the regular macOS font selector dialog:

Make your selections, and in a moment or two Mail.app will update its display.