In the Recent Reviews widget and Upcoming Reviews widget, the images are resized to make them smaller. You can adjust the size of these images with custom CSS.
.widget_ubb_reviews img, .widget_ubb_upcoming_reviews img {
height: 75px;
}
As you can see, the default height is 75px. You can copy that code and change the value to whatever you like.
This code needs to go in a “custom CSS” area. If you have Jetpack, that would be in Appearance > Edit CSS. If you don’t use Jetpack, you can use a plugin like Simple Custom CSS.