It depends on your systems cryptographic hash used to store the passwords.
do sudo cat /etc/shadow
and look for the username followed by $number$
$1$ means you are using MD5
$2$ or $2a$ means you are using blowfish
$5$ means you are using SHA-256
$6$ means you are using SHA-512
for example, sha-512 would give you a 2^512 max length.
sha-256, 2^256th.
In lieu of actual password length issues, I have observed a speed limit when typing my passwords at login.
I am a speedy typist. What I’ve noticed is that if I type my password quickly, I cant login. Instead, if I purposefully type slower, I can login every time.
Recently, I’ve noticed the same issue arising in my mac Mavericks.
Beyond that, I have noticed things like Mint silently enforcing password length and quality standards, that if you were to use the command line, would be apparent, and easier to circumvent. I've had no issues in mint with long passwords, and I have even been able to use hidden characters in passwords without issue.
So try slowing down typing your password at login, it really helped me.
Alternatively, if you’re typing 64 words,for a password, you might be encountering a password timeout limit. as in once you opt to give a password, you have about a minute to enter it, before it times out. At which point you cant get authenticated, even if you do type the correct password.
As for increasing the max password length available, I am aware of a sha-2048
I heard the NSA uses it. Possibly something more exists.
Beyond that, other cryptographic standards exist, which you may look into.