2048 bits is generally considered the minimum size RSA key (assuming you want basically "guaranteed" security, not a "good enough" security). My key is 4096 bits, and the minimum ciphertext using it (encrypting the letter "a") is 905 ASCII-armored characters.That's 6 text messages. Again, it's not impossible or undoable... but the question is why does it absolutely need to be done via SMS?