I believe there are now a few coins which support encrypted messages but they all have issues. The main thing you need to realize about encryption is that both parties typically need to be in possession of the secret encryption key(s). A typical symmetric encryption algorithm such as AES requires that both parties share a common secret. However that's not very useful for encrypting messages in transactions because it requires both parties to be in communication. But if you wanted to send an encrypted message using Cryptonite you could use the AES algorithm to encrypt a message and send it to someone who knows the secret key. There's nothing preventing anyone from doing that right now, but as I said it's not very useful.

Of course Cryptonite and most other altcoins already use public key cryptography (aka asymmetric encryption), and that opens the door for a more convenient message encryption mechanism. Anyone can encrypt a message using the public key associated with some address, then the owner of that address can decrypt the message using their private key. Now the problem is that you can't figure out the full public key just from an address. In order to know what the public key is you either need the person to send you their public key or you need to search the blockchain to find a transaction where that address has already been used as an input (because the public key can be derived from the signature), but it may not have been used previously.

I'm not sure how they implemented message encryption in NXT but I doubt they have solved these fundamental problems. The basic issue is that you can't encrypt a message and expect the other person to know how to decrypt the message unless you're in communication with that person or you have some other way to discover their public key. Thus we haven't implemented message encryption in Cryptonite, because I see no compelling solution to this problem. And to answer your question the limit for Cryptonite messages is 64 characters if I remember correctly. The intended purpose of it is just so people can attach a bit of extra info to the transaction, like maybe an order ID or something. It's not there so people can embed novels into the blockchain.