[alexpeattie]

You can create a more hardened setup by using a 4096 bit RSA key:

  /opt/letsencrypt/letsencrypt-auto certonly --rsa-key-size 4096 --server https://acme-v01.api.letsencrypt.org/directory -a webroot --webroot-path=$DIR -d $DOMAINS 

...and using the secp384r1 curve for ECDHE key exchange:

  # in your nginx.conf
  ssl_ecdh_curve secp384r1;

Arguably, the real state of the art is to use an ECDSA certificate. Let's Encrypt recently started supported them, they offer a equivalent level of security to RSA at much lower bit lengths (a 384 bit ECDSA key is considered equivalent to a 7680 bit RSA key) and a few recent TLS vulnerabilities (like DROWN) have targeted implementation details of RSA.

[lorenzhs]

4096 bit RSA keys offer very little additional security (2048 is plenty for at least the next few years, and with a certificate that's valid for 90 days, there's practically no risk - you can rotate the key rather easily if something bad comes along), but has a fairly big impact on performance and battery life, especially on mobile devices.