@danielgc, Your simple style solution within the action admin_head, was all I needed. You can take things further by tracking down specific iframes on your posts and pages like so…
add_action('admin_head', 'admin_styles');
function admin_styles() {
?>
<style>
iframe[id^='wysiwyg-acf-field-favicon_'] {
min-height:40px;
}
</style>
<?php
}
?>
In my case, I’m targeting only the favicon fields on my WP admin pages, to have a height adjustment.
