To increase the size of the image in the Divi WordPress theme testimonial. Add the following code to the Custom CSS field in ePanel.

body .et_pb_testimonial .et_pb_testimonial_description .et_pb_testimonial_description_inner {
width: 100% !important;
}

.et_pb_testimonial_portrait {
float: left;
height: 180px !important;
width: 180px !important;
}