Hi,

You should paste this code into the function.php of the child theme or via the CodeSnippet plugin here: https://wordpress.org/plugins/code-snippets/

function penci_force_fullsize_images($out, $id, $size) {
    // Check if the image ID is valid
    if (!is_numeric($id) || $id <= 0) {
        return $out; // Return original if invalid ID
    }

    // Check if we already have a valid output. If we do, we should not change it.
    if(is_array($out) && !empty($out[0])) {
        return $out;
    }

    // Attempt to get the full-size image URL
    $image_url = wp_get_attachment_image_src($id, 'full');

    // If a full-size image URL is found, return it
    if ($image_url) {
        return array($image_url[0], $image_url[1], $image_url[2], true);
    }

    // If no full-size image is found, return the original output
    return $out;
}

add_filter('image_downsize', 'penci_force_fullsize_images', 10, 3);

Regards,
PenciDesign.

Hi,

Please go to Appearance → Customize → General Options → Manage Image Sizes From This Theme, then check all the "Disable image size..." options in this section.

After that, clear your site cache and check again.

Regards,
PenciDesign.