How long should a password be, for filesystem encryption?

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http://serverfault.com – I want to encrypt my filesystem with encfs, but I don't know how long my password should be. I'm gonna be using AES 256, with blocks of 4096 bits. How many guesses per second can I expect an attacker can make on files encrypted with that? I'm worried that this might depend on the size of the smallest encrypted file. Any thoughts? (HowTos)