What is the largest password/passphrase size for a 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.