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.
I'm not a programmer to understand the open source code... But do all brainwallet sites use the same algorithm to convert the passphrase to a private key and address? In other words, does a particular passphrase always generate the same private key on all sites?