Bootstrap class: .form-control-sm

<input class="form-control form-control-sm" type="text" placeholder=".form-control-sm">

Preview

Sass source

/* _forms.scss:141 */
.form-control-sm {
  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

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