Bootstrap class: .modal-sm

<!-- Button trigger modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target=".bd-example-modal-sm">
  Small modal
</button>

<!-- Modal -->
<div class="modal fade bd-example-modal-sm" tabindex="-1" role="dialog"
     aria-labelledby="mySmallModalLabel" aria-hidden="true">
  <div class="modal-dialog modal-sm">
    <div class="modal-content">
      ...
    </div>
  </div>
</div>

Preview

More in Bootstrap Modal