Rather than attempt this with ACF, I would do this with CSS. You can wrap the field in a div, like this:
<div class="my-wysiwyg">
<?php the_field('my_wysiwyg');?>
</div>
Then just apply the styling to the last <p> tag with CSS:
.my-wysiwyg p:last-child {
font-size: 30px;
}
Does that help?