1. .modal-sm

<div class="modal">
    <div class="modal-dialog modal-sm" role="dialog" aria-hidden="true">
        <div class="modal-content">
            ...
        </div>
    </div>
</div>
<div class="modal-backdrop" aria-hidden="true"></div>
2. .modal-lg

<div class="modal">
    <div class="modal-dialog modal-lg" role="dialog" aria-hidden="true">
        <div class="modal-content">
            ...
        </div>
    </div>
</div>
<div class="modal-backdrop" aria-hidden="true"></div>
3. .modal-xl

<div class="modal">
    <div class="modal-dialog modal-xl" role="dialog" aria-hidden="true">
        <div class="modal-content">
            ...
        </div>
    </div>
</div>
<div class="modal-backdrop" aria-hidden="true"></div>