bootstrap4.css
@media (min-width: 576px) {
  .modal-dialog {
    max-width: 500px;
    margin: 1.75rem auto;
  }
  .modal-dialog-scrollable {
    max-height: calc(100% - 3.5rem);
  }
  .modal-dialog-scrollable .modal-content {
    max-height: calc(100vh - 3.5rem);
  }
  .modal-dialog-centered {
    min-height: calc(100% - 3.5rem);
  }
  .modal-dialog-centered::before {
    height: calc(100vh - 3.5rem);
  }
  .modal-sm {
    max-width: 300px;
  }
}
■モーダル:modal:小さなサイズのモーダル modal-sm

  <!-- 切替ボタン -->
  <button type="button" class="btn btn-secondary" data-toggle="modal" data-target="#smallModal">モーダルボタン(modal-sm)</button>
  <!-- モーダル -->
  <div class="modal fade" id="smallModal" tabindex="-1" role="dialog" aria-labelledby="smallModalLabel" aria-hidden="true">
    <!-- モーダルのダイアログ本体 -->
    <div class="modal-dialog modal-sm" role="document">
      <div class="modal-content"> 
        <div class="modal-header"> 
          <h5 class="modal-title" id="smallModalLabel">モーダルのタイトル</h5>
          <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">×</span></button>
        </div>
        <div class="modal-body">モーダルの本文が入ります。</div>
        <div class="modal-footer"> 
          <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
        </div>
      </div>
    </div>
  </div>