Google slides gives you the ability to embed Youtube videos directly from a URL or by searching Youtube within the google slides editor. Open a google slides presentation and click on the ‘Insert’ menu followed by ‘video’.
This will open a box in which you can either paste in a direct link to a YouTube video or search YouTube directly from this dialogue box. Once you select a video it will be embedded into the Slide presentation.
Once you have inserted a video you can change the size of the player using the handles on the corners of the video.