Blockchain transaction message length limit When I perform a transaction on the blockchain, I can add a message to the transaction, which will then exist forever in the blockchain.

Say for example I wanted to use the blockchain to store digital assets...

Could this type of information be encrypted and stored as a transaction message, and if so, is there any limit to the message length of a transaction?
This message exists only on blockchain.info site. It is not a part of bitcoin protocol. You should not think, that it will exists forever, because blockchain.info is  third-party-service

Point your view to a OP_RETURN output scripts. http://bitzuma.com/posts/op-return-and-the-future-of-bitcoin/