If you run a command like

sudo certbot certonly -d example.org --standalone --rsa-key-size 4096

it will result in a renewal configuration file containing the following renewal parameters:

rsa_key_size = 4096
authenticator = standalone

If you then run

sudo certbot certonly -d example.org --standalone --reuse-key --force-renewal

rsa_key_size is dropped from the renewal configuration file despite it still being a 4096 bit key.

I think we should preserve this value.