Inline elements usually cannot have width by default. However, there is a way to get what you want through the HTML editor. The code needs to be wrapped in a <div> element with some extra code. Before doing this, make sure you have put the link to the document into the page.
Just in case you have not worked with the HTML editor, here is my usual tip. If you have a bunch of content, go to the spot directly above/below where you want the PP. Using the Insert menu in the editor, insert a Horizontal Line. Then click on the < > in the lower right corner below the editor. You will likely be in the Pretty HTML version. If working with HTML is new, click on Switch to raw HTML editor in the lower left below the editor. Then do CTRL-F to find <hr>. This is that horizontal line. You should see code either above or below it that shows the PowerPoint file. It likely will show up with some <p> and <a class...." codes at the start. Edit the code to look like this:
<div style="display: absolute; width: 50%;">
<p><a class.....> {blah blah blah} </a></p>
</div>
The <div> line will set you up to show everything as a block that is 50% width. Adjust that value as desired. On my end, it shows the top of the second slide and a has a horizontal slide bar, but it does restrict the size down to what you were describing.
I have noticed that once it has loaded, it does not seem to take as long on future refreshes. Mine has 18 slides and it loads before I can even scroll down to it on the page I tested this on.