Mail.app Won't Change Message Font Size
Reviewing this thread in Apple community discussions, it seems that:

a) incoming email font/color/size is determined by what the sender specified;

b) the receiver's message list display is determined by what's set in the receiver's Mail > Preferences > Fonts &amp; Colors > Message List Font; and

c) the receiver's reply (or new message) font/color/size is determined by sender's settings in Mail > Preferences > Fonts &amp; Colors > Message Font

What I'm looking for — that a received message be displayed in a font larger than what's specified by the sender — is apparently not available short of Applescript or add-on app.

Sigh.