On AIX 5.3 to change default length of username:
# chdev -l sys0 -a max_logname=9
sys0 changed
# chdev -l sys0 -a max_logname=9
sys0 changed
To check the current length of username:
# lsattr -El sys0 -a max_logname
max_logname 9 Maximum login name length at boot time True
# getconf LOGIN_NAME_MAX 21