How do I change the Gravatar size

Use an add_filter in your theme’s functions.php file to adjust the Gravatar size.

add_filter( 'testimonials_widget_gravatar_size', 'my_testimonials_widget_gravatar_size' );

function my_testimonials_widget_gravatar_size( $size ) {
    $size                       = 120;

    return $size;
}

Default Gravatar size is 96, maximum 512.