When it comes to lib guides and some other websites, you may need to change the size of your embed code manually. To do this, you need to just change the width to whatever size column you need and the height to whatever you desire. So if your column in the guide where you are putting the video is "600" wide, and the page looks best at "1000" height, then the embed code would look like this:
<iframe src="http://www.your_url_here.com" width="600" height="1000" scrolling="yes" frameborder="1"></iframe>
You would just need to change the numbers on the "width" and "height" section of the embed code to fit whatever place you are putting it.