discourse/app/assets/stylesheets/mobile/select-kit/dropdown-select-box.scss

.select-kit {
  &.dropdown-select-box {
    .select-kit-collection {
      max-height: 200px;
      overflow-y: auto;
    }
  }
}

.select-kit.combo-box .select-kit-row {
  padding: 10px;
}