The Extra theme registers various image sizes with WordPress. This helps to display images at the optimal size, but it can also lead to your server filling up with a lot of image sizes which may never be used. By deregistering some or all of the image sizes, you can reclaim space on your server.

You can use the following PHP code to deregister ALL image sizes currently added by Extra:

/* === Deregister image sizes === */
// See: https://extrabooster.com/remove-unwanted-image-sizes-in-extra/

add_action('after_setup_theme', 'ebc_deregister_image_sizes', 11);

function ebc_deregister_image_sizes() {
    remove_image_size('extra-image-huge'); // 1280x768
    remove_image_size('extra-image-single-post'); // 1280x640
    remove_image_size('extra-image-medium'); // 627x376
    remove_image_size('extra-image-small'); // 440x264
    remove_image_size('extra-image-square-medium'); // 440x440
    remove_image_size('extra-image-square-small'); // 150x150
    remove_image_size('et-builder-post-main-image'); // 400x250
    remove_image_size('et-builder-post-main-image-fullwidth'); // 1080x675
    remove_image_size('et-builder-portfolio-image'); // 400x284
    remove_image_size('et-builder-portfolio-image-single'); // 1080x9999
    remove_image_size('et-builder-gallery-image-portrait'); // 400x516
}

/* === END === */

As this is PHP code, you can add it to the functions.php of a child theme, or using a plugin such as Code Snippets. Please take a backup prior to adding PHP code to your site, so that you can restore the original version if necessary.

For any sizes you want to keep, just delete the corresponding line from the code above. It probably makes sense to keep any sizes that are widely used on the site.

The code above won’t remove the default image sizes added by WordPress itself. It will only apply to images uploaded in the future – to apply the changes to existing images, you should be able to do so with a plugin such as Regenerate Thumbnails.