11.9. Which key type/size should I choose for my key pair? Which is best?

There is no such thing as “the best key.” All choices have consequences and trade-offs. You might feel that a 4096-bit RSA key is safer, but the person you're sending email to might be trying to read it on a PDA which takes over a minute to decrypt each message. You might decide to use SHA-1 because it's widely supported in OpenPGP implementations, but SHA-1 has some mathematical flaw and does not offer long-term security. Finding precisely the optimal set of consequences and trade-offs is a very subtle thing, and the perfect set for you will probably not be the same for anyone else.

The IETF OpenPGP Working Group has spent over a decade looking at which choices offer an excellent balance of speed, safety, and compatibility for the vast majority of users. Their opinions have evolved over time to take into account the technology and threats of the day. The people of the GnuPG project are active participants in the Working Group, and as such GnuPG implements the Working Group's recommendations

Therefore, the best advice we can give is to stick to Enigmail's defaults. They are not perfect, because no two people have the same definition of perfection.
However, the defaults are excellent for the overwhelming majority of users.