increase-editor-font.php

<?php
/*
Plugin Name: Increase Editor Font Size
Description: This plugin will increase the size of the font in the editor only for easier reading and editing.
*/

function plugin_mce_css( $mce_css ) {
        if (! empty( $mce_css ) )
                $mce_css .= ',';

        $mce_css .= plugin_dir_url( __FILE__ ) . '/editor.css';

        return $mce_css;
}

add_filter( 'mce_css' , 'plugin_mce_css' );