What is the largest password/passphrase size for a wallet.dat file? Suppose your wallet.dat file is copied to the wild.  

I am working on methods for creating very strong (512bits, hopefully much more) passphrases that would make it unfeasible to either use rainbow tables or to try brute force it.

Ideally, even if: 

i) one's passphrase is simple; and
ii) the wallet.dat file is copied...

...an adversary would have to employ giant computing power to try to unlock it.  

Since I couldn't get it from the documentation, I wonder if anyone knows how large (in, say, bytes) a passphrase/password can be.   

related question: How does the Bitcoin client encrypt the wallet.dat file?
So just like Nick said, If the encryption is done using 256 bits then creating a passphrase which is 512 bits long would be the same as creating a passphrase which is 256 bits long (as long as its random).

You should look into brainwallets as they provide a way to create strong passwords which are also easily memorable since all you have to memorize is a 3 line poem.