Resize observer✓Baseline · Widely available

const observer = new ResizeObserver((entries) => {
  for (const entry of entries) {
    const { inlineSize } = entry.contentBoxSize[0];
    entry.target.dataset.size = inlineSize > 600 ? "large" : "small";
  }
});

observer.observe(document.querySelector(".chart"));

if ("ResizeObserver" in globalThis) {
  // ResizeObserver is available
} else {
  // load a polyfill or use a fallback
}