Edit in JSFiddle

<h1>100% ширина контейнера</h1>

<figure>
  <iframe src="https://www.youtube.com/embed/4Fqg43ozz7A"></iframe>
</figure>

<h1>75% ширина контейнера</h1>

<div class="figure-75">
  <figure>
    <iframe src="https://www.youtube.com/embed/4Fqg43ozz7A"></iframe>
  </figure>
</div>

<h1>50% ширина контейнера</h1>

<div class="figure-50">
  <figure>
    <iframe src="https://www.youtube.com/embed/4Fqg43ozz7A"></iframe>
  </figure>
</div>

body {
  font: 600 14px/24px "Open Sans", "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", Sans-Serif;
  margin: 0;
}
h1 {
  color: #9799a7;
  font-size: 14px;
  margin-bottom: 6px;
}
figure {
  height: 0;
  margin: 0 0 24px 0;
  padding-bottom: 56.25%; /* 16:9 */
  position: relative;
  width: 100%;
}
.figure-75 {
  width: 75%;
}
.figure-50 {
  width: 50%;
}
iframe {
  border: 0;
  height: 100%;
  left: 0;
  position: absolute;
  top: 0;
  width: 100%;
}