<select class="form-select" aria-label="Default select example">
  <option selected="">Open this select menu</option>
  <option value="1">One</option>
  <option value="2">Two</option>
  <option value="3">Three</option>
</select>

<select class="form-select form-select-lg mb-3" aria-label=".form-select-lg example">
  <option selected="">Open this select menu</option>
  <option value="1">One</option>
  <option value="2">Two</option>
  <option value="3">Three</option>
</select><select class="form-select form-select-sm" aria-label=".form-select-sm example">
  <option selected="">Open this select menu</option>
  <option value="1">One</option>
  <option value="2">Two</option>
  <option value="3">Three</option>
</select>