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<body style="margin:0px;padding:0px;overflow:hidden">
<iframe src="http://www.youraddress.com" frameborder="0" style="overflow:hidden;height:100%;width:100%" height="100%" width="100%"></iframe>
</body>
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<div>
<div style="position:relative;padding-top:56.25%;">
<iframe src="https://www.youtube.com/embed/nckseQJ1Nlg" frameborder="0" allowfullscreen
style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe>
</div>
</div>
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<body style="margin:0px;padding:0px;overflow:hidden">
<iframe src="http://www.youraddress.com" frameborder="0" style="overflow:hidden;overflow-x:hidden;overflow-y:hidden;height:100%;width:100%;position:absolute;top:0px;left:0px;right:0px;bottom:0px" height="100%" width="100%"></iframe>
</body>
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#IFRAME_ID {
width: 100%!important;
height: 100%!important;
}
<iframe src="http://www.youraddress.com" id="IFRAME_ID"></iframe>
#####################
If still not working you can try solution from on the solutions from the link
https://stackoverflow.com/questions/819416/adjust-width-and-height-of-iframe-to-fit-with-content-in-it
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<!DOCTYPE html>
<html lang="en">
<head>
<title>HTML Full-screen iframe (100% Height and Width)</title>
<style>
body{
margin: 0; /* Remove default margin */
}
iframe{
display: block; /* iframes are inline by default */
height: 100vh; /* Set height to 100% of the viewport height */
width: 100vw; /* Set width to 100% of the viewport width */
border: none; /* Remove default border */
}
</style>
</head>
<body>
<iframe src="html/hello.html"></iframe>
</body>
</html>
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#IFRAME_ID {
width: 100%!important;
height: 100%!important;
}
<iframe src="http://www.youraddress.com" id="IFRAME_ID"></iframe>
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<script>
function resizeIframe(obj) {
obj.style.height = obj.contentWindow.document.documentElement.scrollHeight + 'px';
}
</script>