js

addEventListener("resize", (event) => {});

onresize = (event) => {};

html

<p>Resize the floating video window to fire the <code>resize</code> event.</p>
<p>Window height: <span id="height"></span></p>
<p>Window width: <span id="width"></span></p>
<video id="video" src="" muted autoplay></video>

js

const video = document.querySelector("#video");
const heightOutput = document.querySelector("#height");
const widthOutput = document.querySelector("#width");

function resize(evt) {
  heightOutput.textContent = evt.target.height;
  widthOutput.textContent = evt.target.width;
}

video.requestPictureInPicture().then((pictureInPictureWindow) => {
  pictureInPictureWindow.onresize = resize;
  // or
  pictureInPictureWindow.addEventListener("resize", resize);
});