Why isn't add_image_size('event-secondary-image-thumbnail', 250, 150) working?

I am using a plugin called multi post thumbnails so that I can have more than one featured images for each post. I have a custom post type called event. The class multiPostThumbnails exists . it displays the images, it just doesn’t restrict them to 250 by 150 pixels.

How can I make this work, so the images are 250 by 150 pixels only? Thanks!

if (class_exists('MultiPostThumbnails')) {

new MultiPostThumbnails(array(
'label' => 'Secondary Image',
'id' => 'secondary-image',
'post_type' => 'event'
 ) );

 }


function parent_theme_setup() {
    add_image_size('event-secondary-image-thumbnail', 250, 150);
}
add_action( 'after_setup_theme', 'parent_theme_setup' );

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