docs: def size(config, accounts=(), day=None, group=None, human=True, region=None):
      """size of exported records for a given day."""
      config = validate.callback(config)
      destination = config.get('destination')
      client = boto3.Session().client('s3')
      day = parse(day)
  
      def export_size(client, account):
          paginator = client.get_paginator('list_objects_v2')
          count = 0
          size = 0
          session = get_session(account['role'], region)
          account_id = session.client('sts').get_caller_identity()['Account']
          prefix = destination.get('prefix', '').rstrip('/') + '/%s' % account_id
          prefix = "%s/%s/%s" % (prefix, group, day.strftime("%Y/%m/%d"))
          account['account_id'] = account_id
          for page in paginator.paginate(
                  Bucket=destination['bucket'],
                  Prefix=prefix):
              for k in page.get('Contents', ()):
                  size += k['Size']
                  count += 1
          return (count, size)
  
      total_size = 0
      accounts_report = []
      logging.getLogger('botocore').setLevel(logging.ERROR)
      with ThreadPoolExecutor(max_workers=16) as w:
          futures = {}
          for account in config.get('accounts'):
              if accounts and account['name'] not in accounts:
                  continue
              futures[w.submit(export_size, client, account)] = account
  
          for f in as_completed(futures):
              account = futures[f]
              count, size = f.result()
              account.pop('role')
              account.pop('groups')
              total_size += size
              if human:
                  account['size'] = GetHumanSize(size)
              else:
                  account['size'] = size
              account['count'] = count
              accounts_report.append(account)
  
      accounts_report.sort(key=operator.itemgetter('count'), reverse=True)
      print(tabulate(accounts_report, headers='keys'))
      log.info("total size:%s", GetHumanSize(total_size))