I'm not sure what the limits are for the text box at the brainwallet.org website, but for the concept of a brainwallet there is no limit.

With the brainwallet concept, you start with a passphrase and then you create a 256 bit digest of the passphrase.  The most common method is to use SHA256.  SHA256 can create a 256 bit digest of any arbitrarily large data set.