Hello tia
using an image with dimensions of 125×165 is the ideal size which should be used although we noticed that some of the images in testimonials are way too large, not in dimensions but in size. you should use compressed images and then upload the compressed image on your wordpress, then the images will load properly and quickly.
if we increase the size of images then they will cover the text space, so it wont be a good option to do although we can increase the image size from the width of 125 to 167px and the height could be increased to 250px, if you want to do it then let us know.

we also logged in to your website and checked the testimonials in customizer and it was working fine but you forgot to select Russell’s testimonial there. we have selected it for you and it appeared, please check.
Thank you