How do I change the default font size for reading the body of messages in Mail 14.0
Mail > Preferences > Fonts & Colors provides selectors for font style, size, usage (list, text body), and color.
Here's the Fonts & Colors preference pane:
To change the text font, style, and size, push the "Select" button that appears at the right end of the line "Message font." 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.