you dont need to change the dimensions of image it will reduce the quality of picture. keep the dimensions as it is. the theme css takes care of the dimensions, if you want to compress the images then simply search on google “image compression online” and select the first link you see and follow the instructions.
here is a link for you though: https://www.imgonline.com.ua/eng/compress-image-size.php
we have added some css for increasing the height and width of testimonials images.