From
https://www.freebsd.org/cgi/man.cgi?query=adduser&sektion=8&manpath=freebsd-release-ports
RESTRICTIONS
username
Login name. The user name is restricted to whatever
pw(8) will
accept. Generally this means it may contain only lowercase char-
acters or digits but cannot begin with the `-' character. Maxi-
mum length is 16 characters. The reasons for this limit are his-
torical. Given that people have traditionally wanted to break
this limit for aesthetic reasons, it has never been of great
importance to break such a basic fundamental parameter in UNIX.
You can change UT_NAMESIZE in <
utmp.h> and recompile the world;
people have done this and it works, but you will have problems
with any precompiled programs, or source that assumes the 8-char-
acter name limit, such as NIS. The NIS protocol mandates an
8-character username. If you need a longer login name for e-mail
addresses, you can define an alias in
/etc/mail/aliases.