discourse/app/assets/stylesheets/common/select-kit/list-setting.scss

.select-kit {
  &.multi-select {
    &.list-setting {
      .select-kit-row.create {
        .square {
          width: 12px;
          height: 12px;
          margin-left: 5px;
        }
      }
    }
  }
}