David Bradshaw and Chris Jacob already suggested using the postMessage approach. And I totally agree, that the proper way of doing things like these.
I just want to post an example, real code that works, in case it'll be a ready answers for some.
On the iframed-side:
<body onload="docResizePipe()"><script>var v = 0;const docResizeObserver = new ResizeObserver(() => { docResizePipe();});docResizeObserver.observe(document.querySelector("body"));function docResizePipe() { v += 1; if (v > 5) { return; } var w = document.body.scrollWidth; var h = document.body.scrollHeight; window.parent.postMessage([w,h], "*");}setInterval(function() { v -= 1; if (v < 0) { v = 0; }}, 300);</script>Note the recursion-blocking mechanics - it was necessary because of apparently a bug in Firefox, but anyways let it be there.
On the parent document side:
<iframe id="rpa-frame" src="3.html" style="border: none;"></iframe><script>var rpaFrame = document.getElementById("rpa-frame");window.addEventListener("message", (event) => { var width = event.data[0]; var height = event.data[1]; rpaFrame.width = parseInt(width)+60; rpaFrame.height = parseInt(height)+60; console.log(event);}, false);</script>Hope it'll be useful.
