JSFiddle - React, Tailwind, and code Playground

HTML

<label>SELECT input with 'xs' class</label>
<select class="form-control input-xs">
  <option>Choose ...</option>
  <option>Option 1</option>
  <option>Option 2</option>
  <option>Option 3</option>
  <option>Option 4</option>
</select>

<br />

<label>Normal SELECT input</label>
<select class="form-control">
  <option>Choose ...</option>
  <option>Option 1</option>
  <option>Option 2</option>
  <option>Option 3</option>
  <option>Option 4</option>
</select>

CSS

@import url('//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css');

.input-xs{
    height: 20px;
    line-height: 1.5;
    font-size: 12px;
    padding: 1px 5px;
    border-radius: 3px;
}

body {
  padding: 10px;
}