Key sizes for AES are 128-bit, 192-bit and 256-bit. Depending on how you pack characters into bits, will determine how many characters that makes up. For example, you could pack 1000 characters into a 128-bit encryption key using something like PBKDF2 or scrypt.
That said, I wasn't aware that Truecrypt only supports 64 character passwords. Assuming that is correct (and I have no reason to doubt you yet), you could always use a keyfile with Truecrypt if you need a longer password, but keep in mind that a randomly chosen, 64 character password is surely infeasible to break any time soon.