code: public static function getMaxUploadSize($max_size = 0)
      {
          $post_max_size = Tools::unformatBytes(ini_get('post_max_size'));
          $upload_max_filesize = Tools::unformatBytes(ini_get('upload_max_filesize'));
          if ($max_size > 0) {
              $result = min($post_max_size, $upload_max_filesize, $max_size);
          } else {
              $result = min($post_max_size, $upload_max_filesize);
          }
  
          return $result;
      }
docs: Get max file upload size considering server settings and optional max value
  
  @param int $max_size optional max file size
  
  @return int max file size in bytes