You'd be surprised how much entropy a relatively short passphrase can have.
Let's assume, for a minute, that 128-bit symmetric encryption was sufficient for the time frame you're interested in. That's a pretty safe assumption; in order to have even a one-in-a-million chance to guess the correct key in 100 years (which is the time frame of your linked question) would require a computer system able to calculate roughly 1.079e23 (that's 108 hexillion) possible keys per second. Even given the exponential pace of technology, it's extremely unlikely that even governments would gain access to that kind of computing power in the next 100 years. So, a hash of that length should be considered adequate.
Now, understand that hashing doesn't increase entropy; if there are only 1 million possibilities for a password used to generate a key, then it doesn't matter if the resulting key length is 32 bits or 256; all an attacker has to do, knowing the million possibilities, is simply try each of those instead of brute-forcing the entire keyspace. Therefore, to ensure that the strength of the encryption can't be sidestepped by guessing a weak password, we should have a passphrase that provides at least 128 bits of entropy. Assuming an entropy calculation of approximately 1.5 bits per letter when using common English dictionary words, and an average of 6.5 letters per dictionary word (by enforcing a minimum word size of 4 letters) the resulting passphrase would only have to be about 13 words long.
That's if the words chosen were completely random, and therefore by knowing each previous word you would still have no clue what the next word would be. It would be difficult, but not impossible, to memorize 13 words and to pass that on to your descendants; you might form a poem out of the words with a system inherent in embedding the actual words of the passphrase with the filler. If your passphrase was itself a meaningful sentence, it would have to be longer because the number of words that would have meaning after any given combination would be reduced, thus reducing entropy of the phrase. You could probably get away with a paragraph of a literary work. The more obscure, the better; pretty much any work of Shakespeare is out, as are any holy texts like the Bible, Qu'ran, Mahabharata, etc. A completely original statement would be best. Either way, if you make the sentence meaningful you need more words; probably closer to 20 or 25 words, not counting words with three letters or less.