I am assuming you are embedding a Google Slide. In your content editor, if you change the view to HTML, you should see something like this:
<p><iframe src="https://docs.google.com/presentation/**********/embed?start=false&loop=false&delayms=3000" width="650" height="396" allowfullscreen="allowfullscreen" webkitallowfullscreen="webkitallowfullscreen" mozallowfullscreen="mozallowfullscreen"></iframe></p>
You can then adjust the width and height numbers. If your black bars are on top and bottom, you would just want to lower the height number (or raise the width number). The code above is pulled directly from one of my teacher's homepages, so those numbers might work for you too.
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