BLOB field key value length issue in custom field schema

I'm getting following error when trying to create custom field.

There was a problem creating field Signature: Exception thrown while performing a schema update. SQLSTATE[42000]: Syntax error or access violation: 1170 BLOB/TEXT column 'field_signature_value' used in key specification without a key length: CREATE TABLE {node__field_signature} ( `bundle` VARCHAR(128) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL DEFAULT '' COMMENT 'The field instance bundle to which this row belongs, used when deleting a field instance', `deleted` TINYINT NOT NULL DEFAULT 0 COMMENT 'A boolean indicating whether this data item has been deleted', `entity_id` INT unsigned NOT NULL COMMENT 'The entity id this data is attached to', `revision_id` INT unsigned NOT NULL COMMENT 'The entity revision id this data is attached to', `langcode` VARCHAR(32) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL DEFAULT '' COMMENT 'The language code for this data item.', `delta` INT unsigned NOT NULL COMMENT 'The sequence number for this data item, used for multi-value fields', `field_signature_value` LONGBLOB NULL DEFAULT NULL COMMENT 'base64 value of signature image', PRIMARY KEY (`entity_id`, `deleted`, `delta`, `langcode`), INDEX `bundle` (`bundle`), INDEX `revision_id` (`revision_id`), INDEX `field_signature_value` (`field_signature_value`) ) ENGINE = InnoDB DEFAULT CHARACTER SET utf8mb4 COMMENT 'Data storage for node field field_signature.'; Array ( )

As my understand I need to mention length for the key value. But how could I mention length for blob field?

  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return array(
      'columns' => array(
        'value' => array(
          'type' => 'blob',
          'size' => 'big',
          'not null' => FALSE,
          'description' => 'base64 value of signature image',
        ),
      ),
      'indexes' => array(
        'value' => array('value'),
      ),
    );
  }