Setting a default font and font-size in TinyMCE

editor_plugin.js

(function() {

    tinymce.create('tinymce.plugins.CustomSetFontPlugin', {

        /**
         * Initializes the plugin, this will be executed after the plugin has been created.
         * This call is done before the editor instance has finished it's initialization so use the onInit event
         * of the editor instance to intercept that event.
         *
         * @param {tinymce.Editor} ed Editor instance that the plugin is initialized in.
         * @param {string} url Absolute URL to where the plugin is located.
         */
        init : function(ed, url) {
            ed.onLoadContent.add(function(ed, o) {

                //CSS changes you can make - This is for user display purposes and does not actually get sent with the email.
                // ed.getBody().style.fontSize = '12pt';
                //ed.getBody().fontFamily = 'arial';

                //This will embed the font tags into the HTML body so that it is sent with the email
                ed.setContent('<p><span style="font-family: arial,helvetica,sans-serif; font-size: small;">&nbsp;</span></p>');
            });
        },

        /**
         * Creates control instances based in the incomming name. This method is normally not
         * needed since the addButton method of the tinymce.Editor class is a more easy way of adding buttons
         * but you sometimes need to create more complex controls like listboxes, split buttons etc then this
         * method can be used to create those.
         *
         * @param {String} n Name of the control to create.
         * @param {tinymce.ControlManager} cm Control manager to use inorder to create new control.
         * @return {tinymce.ui.Control} New control instance or null if no control was created.
         */
        createControl : function(n, cm) {
            return null;
        },

        /**
         * Returns information about the plugin as a name/value array.
         * The current keys are longname, author, authorurl, infourl and version.
         *
         * @return {Object} Name/value array containing information about the plugin.
         */
        getInfo : function() {
            return {
                longname : 'CustomSetFont plugin',
                author : 'Jerry Clark',
                authorurl : 'http://twitter.com/geraldclark',
                infourl : '',
                version : "1.0"
            };
        }
    });

    // Register plugin
    tinymce.PluginManager.add('customsetfont', tinymce.plugins.CustomSetFontPlugin);
})();

tinyMCEDefaultConfig.php

<?php

    $defaultConfig = array(
        //override 'default' plugin setup - does not set plugins for 'email_compose' or 'email_compose_light'
        'plugins' => 'advhr,insertdatetime,table,preview,paste,searchreplace,directionality,customsetfont',
    );