Video size | WordPress.org

Hi @nicobonder

Please go to Appearance > Customize > Additional CSS section in your WordPress Admin Area, and paste the below provided CSS there:

.wp-block-embed__wrapper {
    float: none;
    clear: both;
    width: 100%;
    position: relative;
    padding-bottom: 56.25%;
    padding-top: 25px;
    height: 0;
}
iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

It should work. 🙂