custom-select-sm

bootstrap5.css

bootstrap5.css
.input-group-sm > .form-control:not(textarea),
.input-group-sm > .custom-select {
  height: calc(1.5em + 0.5rem + 2px);
}
bootstrap5.css
.input-group-sm > .form-control,
.input-group-sm > .custom-select,
.input-group-sm > .input-group-prepend > .input-group-text,
.input-group-sm > .input-group-append > .input-group-text,
.input-group-sm > .input-group-prepend > .btn,
.input-group-sm > .input-group-append > .btn {
  padding: 0.25rem 0.5rem;
  font-size: 0.875rem;
  line-height: 1.5;
  border-radius: 0.2rem;
}
bootstrap5.css
.input-group-lg > .custom-select,
.input-group-sm > .custom-select {
  padding-right: 1.75rem;
}
bootstrap5.css
.custom-select-sm {
  height: calc(1.5em + 0.5rem + 2px);
  padding-top: 0.25rem;
  padding-bottom: 0.25rem;
  padding-left: 0.5rem;
  font-size: 0.875rem;
}

html bootstrap5 Sample

プルダウンメニュー 小：custom-select-sm

  <form>
    <select class="custom-select custom-select-sm">
      <option selected>選択してください</option>
      <option value="1">1</option>
      <option value="2">2</option>
      <option value="3">3</option>
    </select>
  </form>