Selective use of posts_morelink

Is it possible to have the “posts_morelink” output “Read More” only when an excerpt exceeds the designated character limit (excerpt_size=55)? The bulk of my posts are testimonials that are less than 55 characters, and don’t need the “Read More” link. In certain instances there are longer testimonials that do require the read more link.

Thanks in advance!

https://wordpress.org/plugins/list-category-posts/