In Mac OS X Mail, you can choose a default font for both reading (plain text) and composing emails. Of course, you can also specify a default size.
Change the Default Mac OS X Mail Message Font
To specify a default font face and size for composing (and reading) mail in Mac OS X Mail:
- Select Mail | Preferences... from the menu.
- Go to the Fonts & Colors category.
- Select the desired font under Message font:.
- Specify the desired font size under Size: in the same line.
- Go to the Composing category.
- Make sure Rich Text is selected under Composing: Message Format:.
- Under Responding:, prefarably also check Use the same message format as the original message. This means people who send you plain text messages will get emails back in plain text from you — your default font is not used for them, but this is probably what they prefer.
- Close the preferences window.