This is a macOS problem. OpenSSL on Linux has extensive man pages and a --help switch.
E.g. OpenSSL on OpenBSD does have a man page. But unfortunately no --help, at least on my older version.
Also, on OpenBSD his example of generating an RSA key creates one with a 2048 bit modulus.
> (bso) larrik@local:~/bso$ openssl --help > openssl:Error: '--help' is an invalid command.
It lists a bunch of valid commands, though. Of course, you can't do `openssl help genrsa` or anything sensible like that.
$ openssl --help
Invalid command '--help'; type "help" for a list.