Zerif Lite Testimonials Widget Image size

I’ve done a search and cannot find anything on the topic so here goes.

I wondered if there is a way to increase the image size on the Zerif Lite Testimonials Widget.

I found this in functions.php but increasing the max-width doesn’t seam to do anything. Perhaps it is the wrong place altogether? Maybe it relies on size of the widget as a whole?

<img class="custom_media_image_testimonial" src="' . $instance['image_uri'] . '" style="margin:0;padding:0;max-width:100px;float:left;display:inline-block"

Thanks for any help.