Understanding OpenPGP 33
default to 2048-bit keys. A 2048-bit symmetric key suffices to
provide robust security for the next several years, unless your
attacker has quantum computers or one of those ultrasecret
custom-built, code-busting machines the NSA is rumored
to have.
Increasing the key size increases the amount of work
needed to process your key—not just the amount of work
needed to send encrypted emails but also the amount of work
others must do to read them. It also increases the amount of
work the bad guys have to do to break your key, however. Stick
with the defaults, unless you know that everyone you will ever
exchange encrypted messages with has sufficient computing
power to decrypt your messages without having to take a coffee ...