If you know, please also provide relevant documentation.

UPDATE (2025-02-02T06:06Z): I did some brute-force testing, and, at least for sh.itjust.works, it seems that the maximum username length is 50, and the maximum password length is 60 [1].

References
  1. “Sign Up”. sh.itjust.works. Lemmy. Accessed: 2025-02-02T08:49Z. https://sh.itjust.works/signup.
    • When creating an account on sh.itjust.works, the sign-up form will throw this error if the provided password is greater than 60 characters in length.
  • Andrew@piefed.socialEnglish
    2·
    3 hours ago

    It seems to be set per-instance - you can find out through their APIs:

    curl -L http://lemmy.world/api/v3/site | jq -r .site_view.local_site.actor_name_max_length (26)

    curl -L http://sh.itjust.works/api/v3/site | jq -r .site_view.local_site.actor_name_max_length (50)