SQL command to delete unused image sizes?

The regenerate command will do what you want. You just need to remove your unwanted image sizes before you run it.

There are a few different methods needed to get at all of the image sizes that core has put in place. This code snippet removes medium_large, 1536×1536, 2048×2048, and “-scaled”.

add_filter('intermediate_image_sizes', 'wpsf_remove_medium_large_image_size');
function wpsf_remove_medium_large_image_size($sizes)
{
  return array_diff($sizes, array('medium_large'));
};

add_action('init', 'wpsf_remove_large_image_sizes');
function wpsf_remove_large_image_sizes()
{
  remove_image_size('1536x1536');
  remove_image_size('2048x2048');
}

add_filter('big_image_size_threshold', '__return_false');

With that in place, new uploads to the media library will not have those image sizes generated. And you’re ready to delete the unwanted sizes that are already sitting on your server. I recommend the “delete old unregistered sizes” feature of the Regenerate Thumbnails plugin.

What’s nice about the plugin, as opposed to the WP CLI regenerate command, is that it reports the currently active sizes in a list so that you can check that the above code is working before you regenerate. And, of course, you should take a backup of your uploads folder beforehand.

You’re welcome!

Thank you for sharing that post. Elliott’s code snippet is cleaner. I’ve reproduced it here for posterity.

// Disable generated image sizes
function wpsf_disable_image_sizes($sizes) {
    unset($sizes['thumbnail']);
    unset($sizes['medium']);
    unset($sizes['large']);
    unset($sizes['medium_large']);
    unset($sizes['1536x1536']);
    unset($sizes['2048x2048']);
    return $sizes;
}
add_action('intermediate_image_sizes_advanced', 'wpsf_disable_image_sizes');

// Disable scaled image size
add_filter('big_image_size_threshold', '__return_false');

Although if one is using the Regenerate Thumbnails plugin, I recommend my version. It makes the plugin’s sizes listing match what will happen when pressing the Regenerate Thumbnails button.