Don't want to learn Bootstrap classes? Go to Bootstrap builder for busy developers.

Bootstrap class: .input-group-lg

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

Preview

Large

Sass source

/* _input-group.scss:129 */
.input-group-lg > .input-group-append > .btn {
  height: $input-height-lg;
  padding: $input-padding-y-lg $input-padding-x-lg;
  font-size: $font-size-lg;
  line-height: $input-line-height-lg;
  @include border-radius($input-border-radius-lg);
}

Uses variables

/* _input-group.scss:129 */
$input-height-lg
$input-padding-y-lg
$input-padding-x-lg
$font-size-lg
$input-line-height-lg
$input-border-radius-lg