Ask HN: Best cellular device for verbose real-time IM?

From typical cellphone-size where the on-screen keyboard is impractically small, to a standard iPad where the keyboard is ok if you're used to it but the device is not exactly compact, I'm looking for the smallest device that'll reliably do the job. I have not tried 7" tablets, phablets, nor obviously the iPad mini. What works for you in a situation where chats can get pretty verbose yet as engineers we aren't exactly longing for voice and certainly not video conferencing :) ?