Bootstrap class: .input-group-sm

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<div class="input-group input-group-sm">
  <div class="input-group-prepend">
    <span class="input-group-text" id="inputGroup-sizing-sm">Small</span>
  </div>
  <input type="text" class="form-control" aria-label="Small" aria-describedby="inputGroup-sizing-sm">
</div>

Preview

Small

Sass source

/* _input-group.scss:141 */
.input-group-sm > .input-group-append > .btn {
  height: $input-height-sm;
  padding: $input-padding-y-sm $input-padding-x-sm;
  font-size: $font-size-sm;
  line-height: $input-line-height-sm;
  @include border-radius($input-border-radius-sm);
}

Uses variables

/* _input-group.scss:141 */
$input-height-sm
$input-padding-y-sm
$input-padding-x-sm
$font-size-sm
$input-line-height-sm
$input-border-radius-sm

More in Bootstrap Form Input Groups