Below is a script that should demonstrate all of the commands that one needs for public key encryption with very long files. That is, I use the public / private keys to encrypt a symmetric password that in turn can encrypt arbitrarily large data.
The files in "stage" are the ones that you'd attach to an email, as I remember. For now, I'm posting this as-is. I'm not certain I have all this right, but it's a good basis at least.
As I remember, in Cryptonomicon, 4096 bits was cited as lasting for decades or centuries, but I leave that as an exercise to the reader. 16384 bits was taking perhaps over a minute to generate. I didn't wait for it. 8092 bits is quick enough and more than 4096.