<!-- Small modal -->

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

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

<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>