The font style and font size come from your template stylesheet.

Which template are you using?

There are 2 ways to solve this:

1. Create a specific stylesheet for the editor, that imports the template stylesheets and adds an override for the font size - https://www.joomlacontenteditor.net/support/tutorials/editor/create-a-custom-editor-stylesheet
2. Add an override to one of the Astroid Framework stylesheets, eg: - media/templates/site/astroid_template_one/css/custom.css - using an editor specific selector, eg:
   
   

   .mceContentBody * {
       font-size: 14px;
       font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
   }