To do this, apply CSS styling. The paragraph has CSS ID openinghours_adminlabel, and the chosen time has CSS ID openinghours_email_timechosen.
One way to do this is by adding a CSS rule via a code filter. e.g.:
add_action('woocommerce_email_header', function ($email_heading, $email) {
// Adds CSS to "new order" email only
if ($email->id != 'new_order') return;
?>
<style type="text/css">
p#openinghours_adminlabel { font-size: 28px; }
</style>
<?php
}, 20, 2);
Posted in: WooCommerce Opening Hours and Delivery Times