Yup...the next Nxt release (

cheesy ) will allow a 40 kB payload per message, either clear or encrypted.
Hmm.....
Really? What would that do to the blockchain (i.e., bloat)?
the blockchain will become more and more heavier
How to implement Blockchain 'trimming' has already been discussed and also referenced in the voting section of NRS version 1.5.0e. I think if you want a message to persists beyond each trim, there will be higher fees. But not sure yet so don't quote me