iframe 100% -лучший способ

.videoWrapper {
 position: relative;
 padding-bottom: 56.25%; /* 16:9 */
 padding-top: 25px;
 height: 0;
}
.videoWrapper iframe {
 position: absolute;
 top: 0;
 left: 0;
 width: 100%;
 height: 100%;
}