There may be a misunderstanding here.
"Selecting the correct kmer size for kmergenie analysis" implies that kmergenie would perhaps require user assistance?
This is precisely the opposite, it is designed to choose k for you. Just run kmergenie without any parameter, and it will explicitly print what it thinks is a good kmer size. Most of the time, the value is accurate.
Additionally, for unusual cases, you can take a look at the generated HTML report to make a more informed decision.