Youtube video embed size, how to make larger?

Youtube videos used to embed the full width of a post, after an update they now embed smaller. How to change back to post width? Thanks.

Quote from Ramiru

This (https://css-tricks.com/fluid-width-video/) :

Sass (Scss)

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

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Thanks. What do I do with this code?

Worked nice, thank you.