<!-- using grid -->
<div class="row">
  <div class="col-sm-8">
    <div class="card">...</div>
  </div>
</div>

<!-- using utilities -->
<div class="card w-25">...</div>
<div class="card w-50">...</div>
<div class="card w-75">...</div>

<!-- using custom css -->
<div class="card" style="width: 18rem;">...</div>