Image size

That would be a nice feature but the plugin does not offer that setting.

It uses the get_the_post_thumbnail function with a default size of thumbnail.

You can change it manually by editing testimonials.php, searching for the first occurrence of get_the_post_thumbnail and changing thumbnail to medium, large, full, or a custom size like array(100, 100).

Not quite. Notice the difference between set and get.

Line 359 in function gct_single_testimonial:

$display .= '<div class="photo">'.get_the_post_thumbnail($testimonial->ID, 'thumbnail').'</div>';