gitlab-org--gitlab-foss/app/assets/stylesheets/generic/gfm.scss

/**
 * Styles that apply to all GFM related forms.
 */
.issue-form, .merge-request-form, .wiki-form {
  .description {
    height: 20em;
  }
}

.wiki-form {
  .description {
    height: 26em;
  }
}

.milestone-form {
  .description {
    height: 14em;
  }
}