How do I set the Mail app's default font running under Mavericks? Mail says its default font is 12pt Helvetica. 

How do I change the default to something more readable, such as 14 pt Georgia?

Appreciated.
Open Mail Preferences and then the Font setting.

You have 2 settings.

Set the Font for the List view and set the font for the message body.

When you click Select and new widow will open showing the font choices.

This will change your mail display.