To cite effectsize in publications use:

  Ben-Shachar M, Lüdecke D, Makowski D (2020). effectsize: Estimation
  of Effect Size Indices and Standardized Parameters. Journal of Open
  Source Software, 5(56), 2815. doi: 10.21105/joss.02815

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{e}ffectsize: Estimation of Effect Size Indices and Standardized Parameters},
    author = {Mattan S. Ben-Shachar and Daniel Lüdecke and Dominique Makowski},
    year = {2020},
    journal = {Journal of Open Source Software},
    volume = {5},
    number = {56},
    pages = {2815},
    publisher = {The Open Journal},
    doi = {10.21105/joss.02815},
    url = {https://doi.org/10.21105/joss.02815},
  }