How do I post a video from Youtube in a big window?

I've read many articles and posts on how to post videos from Youtube on Facebook, and they all say to simply paste the share code or the link in a comment box and click post, and the video will automatically be posted. I've tried this, but what gets posted is a rectangle with the video in a small box next to a larger box with the video description in it, and you have to click on the video to play it. I've seen posts from other people that just have a large window with the video in it, and the video starts playing as soon as I scroll to it. How do I do this?