I recall another user reporting issues once with using the Semplice theme.
The problem seems to be that Semplice loads part of the page via AJAX requests (e.g. not loading the entire page, but only parts of it). Fontsampler libraries are only getting loaded when a WordPress page uses the shortcode. The problem with Semplice thus is that it fetches only part of the page (where Fontsampler is used) and does not trigger this loading of Fontsampler libraries. One thinkable approach would be to force loading the Fontsampler libraries on any page, so that when Semplice loads a partial page with a Fontsampler on it, the libraries are available. For this, you’d want to add these to your theme’s functions.php:
add_action('wp_enqueue_scripts', function () {
wp_enqueue_style('fontsampler-css', '/wp-content/plugins/fontsampler/css/fontsampler-css.css');
wp_register_script('fontsampler-js', '/wp-content/plugins/fontsampler/js/fontsampler.js', array('jquery'), false, false);
});
(Untested code, but it should point you in the right direction).
You might also want to contact Semplice support, since essentially it is their violation of WordPress standards that is the root cause of this problem.
Hope this helps you out,
-Johannes
As for your second question, you set the default size of the text under the Features > Font size (even if you do not use the font size slider feature you can set the initial font size there).