Based on investigation from HELP-39572, there is an undocumented limit on username size in bytes of ~7.99MB. It might be a good idea to document a limit of 7MB or so. Note - there is a documented limit of 10kB only for $external authentication users documented here: https://www.mongodb.com/docs/v4.4/reference/limits/#mongodb-limit-Sessions-and--external-Username-Limit. This limit applies back until at least MongoDB 4.4, and forward until today.