How long's the actual Truecrypt key? Ordinary conversational English has about 3 bits of entropy per character, so 64 characters will have an equivalent strength to a 192-bit key. If Truecrypt uses 128-bit keys that's plenty; if they use 256-bit keys, they ought to allow a longer passphrase.
Of course if you choose truly random characters you have 7 bits of entropy per character, so you would have the same strength with a 28-character password. But which is going to be easier to remember, 28 random characters or 64 characters of ordinary English?
If you use a pass phrase it will be easier to crack with a dictionary attack since at best you will put about 15 words in 64 chars.