Setting Password Length:
By default, Ubuntu requires a minimum password length of 4 characters, these values are controlled in the file /etc/pam.d/common-password, which is outlined below, look for the line having "pam_unix.so" mentioned ...
password [success=1 default=ignore] pam_unix.so nullok obscure min=4 max=8 sha512If you would like to adjust the minimum length to 6 characters, change the appropriate variable to min=6. The modification is outlined below.
password [success=1 default=ignore] pam_unix.so nullok obscure min=6 max=8 sha512