JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width" />
    <link
          rel="stylesheet"
          href="https://cdn.jsdelivr.net/npm/@fancyapps/ui@5.0/dist/fancybox/fancybox.css"
          />
    <style>

      /* For compact mode */
      .fancybox__container.is-compact .fancybox__slide.my-iframe .fancybox__content {
        width: 100%;
        height: 100%;
      }

      /* For all other cases */
      .fancybox__slide.my-iframe .fancybox__content {
        width: 400px;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div>
      <p>
        <a
           href="https://fancyapps.com/iframe.html"
           data-fancybox
           data-type="iframe"
           data-custom-class="my-iframe"
           >
          Custom width and height using CSS
        </a>
      </p>
    </div>
    <script src="https://cdn.jsdelivr.net/npm/@fancyapps/ui@5.0/dist/fancybox/fancybox.umd.js"></script>
    <script>
      Fancybox.bind("[data-fancybox]", {
        Html: {
          autoSize: false,
        },
      });
    </script>
  </body>
</html>