Current MacOS (Mojave, 10.14.5) has a man page for openssl, command line help for openssl, and generates a 2048 bit modulus, and is LibreSSL version 2.6.5 (admittedly nearly a year old.)

    $ openssl genrsa -out private_key.pem
    Generating RSA private key, 2048 bit long modulus
    .................+++
    ........................................+++
    e is 65537 (0x10001)