modal-sm

bootstrap5.css

bootstrap5.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;
  }
}

html bootstrap5 Sample

小さなサイズのモーダル：modal-sm

    <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">
      <!-- 小さなモーダルのダイアログ本体modal-sm -->
      <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</div>
          <div class="modal-footer">
            <button type="button" class="btn btn-secondary" data-dismiss="modal">閉じる</button>
          </div>
        </div>
      </div>
    </div>