Follow How do I change the image size

The default image size is based upon Thumbnail size in Media Settings. If changing that doesn’t work for you, then use add_filter in your theme’s functions.php file to adjust the image size.

add_filter( 'tw_image_size', 'my_testimonials_widget_image_size' );

function my_testimonials_widget_image_size( $size ) {
    $size                       = array( 120, 90 );

    return $size;
}

You can use either a string keyword (thumbnail, medium, large or full) or a 2-item array representing width and height in pixels, e.g. array(32,32).

I recommend putting your theme customizations into a custom-functions.php file and then include that in your theme’s functions.php file via include 'custom-functions.php';.

Image Size Defined?

You might also ensure that your image size is defined if you want WordPress to automatically create that size upon media uploading. See add_image_size for details.

Image Looking Small?

If the code above is in place, but the source images are too small, then your source images are too small. To fix, provide larger source images.