macOS Sierra 10.12.6, Mail.app 10.3,
I can change the size of the message list font in Mail > Preferences > Fonts & Colors, and the change is reflected in Mail.app's message list. However, changes to the message font are not reflected by a change in the message display.
This behavior appears on two computers, a Macbook Pro 7,1 (mid-2010) and a Macbook Air 6,1 (mid-2013). Both computers' software is updated. In all other respects, both computers operate correctly.
Is there a cache or plist to clear or something else to do to allow the message font size to change?