So how do i get a 31 bit key?
That brings up back to your initial post. A challenge of replying to forum entries is to guess the appropriate level of detail for new participants.
I guess you don't know what to do with
openssl genrsa 31 > key.private
It is a pfSense shell command to write a 31 bit key into the file key.private. Do you know how to execute a pfSense shell command?
If I have guessed incorrectly then you will need to explain in more detail the difficulty you are having in following the instructions in the post I previously linked to.