1. Open functions.php file of your theme

2. Put this code to the file

add_action( 'admin_print_styles-post.php', 'my_admin_css' );
add_action( 'admin_print_styles-post-new.php', 'my_admin_css' );
function my_admin_css() {
?>
  <style>
    #editorcontainer textarea#content { 
      font-size:120% !important 
    }
  </style>
<?php
}

3. Reload