make bootstrap cards smaller with output changing his content code example

Example 1: card center in bootstrap

.card {
        margin: 0 auto; /* Added */
        float: none; /* Added */
        margin-bottom: 10px; /* Added */
}

Example 2: bootstrap + cards

<div class="card" style="width: 18rem;">
  <img class="card-img-top" src="..." alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Card title</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
    <a href="#" class="btn btn-primary">Go somewhere</a>
  </div>
</div>