This article explains how to choose a default font for reading plain text and composing emails, and how to choose a default size in Apple's Mail app on Macs running macOS 10.15 Catalina through macOS Sierra (10.12).
Set font size in Apple Mail
To specify a default font and size for composing and reading email in the Mac macOS Mail application, do the following:
Launch the Mail application and select Mail > Preferences from the menu bar.
Select the Fonts & Colors tab in the Preferences screen.
