You need to open your functions.php file and put following code in that file.using following code you can change the font size of editor.

Note: If you are not having knowledge of wordpress code and php then dont use  the code.

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 type=”text/css”>
#editorcontainer textarea#content { font-size: 2em !important }
</style>
<?php

 

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