Responsive embeds automatically adjust to the containing element.
<div class="embed-responsive embed-responsive-16by9">
<iframe class="embed-responsive-item" title="How to paraphrase" src="https://www.youtube.com/embed/oTsE-LRSo_M" allowfullscreen></iframe>
divto contain the embed
embed-responsive-4by3depending on the aspect ratio of the content
embed-responsive-itemto the iframe, video, etc.
That's it. You know have an embed that adjusts it's size based on the containing element.