Changing the default WP editor font and size

You’ll need to add support for your theme (if it does not already have it) for editor styles. See this for more information:

http://codex.wordpress.org/Function_Reference/add_editor_style

Then you’ll just need to create a stylesheet. There are some themes out there that already support editor styles, which might be a good place to start in order to know the class names, etc.