If you have OneDrive or Google Drive connected to your account, that could provide an option. For example, I could make a single set of instructions on a Google Doc. Within a Canvas Page/Assignment, I could use the Apps option (looks like a plug icon) to select Google Drive (LTI 1.3). I would select the document containing the instructions (it will ask to adjust the sharing settings to anyone with the link can view) and embed it onto the page. It will load up in a way that allows me to view it on the page.
Now for a few things it will do. It will likely put it at a Width/Height that is smaller than your window. You may also want to more visually highlight it for students. You will need to into the HTML editor, but the changes only need to be done once. I would suggest testing this on a blank page and getting it the way you want, then leave the page unpublished so you can easily copy and paste whenever needed.
First, you may want to visually mark the space off. This can be easy to do with some Horizontal Rules (Insert Menu) and a Heading. The horizontal rule will show up as a thin black line, but can be adjusted to be thicker and any color you want in HTML. Then use the Google Drive (LTI 1.3) option to embed the document between the horizontal lines. Finally, go into your HTML editor and make adjustments. The horizontal rules will show up as <hr>, while the width and height for the embedded document will be within an iFrame code. Here is an example after making some changes (I stripped out the title and link, so the parts highlighted in orange below can be ignored as that will be automatically generated when you use embed the content):
<hr style="border-top: 3px solid #008896;" />
<h2><strong>Instructions</strong></h2>
<p><iframe style="width: 98%; height: 750px;" title="Document Title" src="link" allowfullscreen="allowfullscreen" allow="geolocation *; microphone *; camera *; midi *; encrypted-media *; autoplay *; clipboard-write *; display-capture *"></iframe></p>
<hr style="border-top: 3px solid #008896;" />
In the code above, copy and paste the entire line for the <hr....> to change the line style. The Blue 3px is the thickness (increase for thicker) and the red #008896 is the color code. Look anywhere online for HTML color codes, pick the color you want, and copy the code to this spot. Do it for both <hr...> lines.
For the content from Google (or OneDrive), I've changed the width from the 750px default value to 98% (marked in green). I've noticed in code that full screen content in Canvas often defaults to 98.xyz, so I arbitrarily just did 98% for this example. This will help fill the view. Change the height as needed, but make sure to keep it as px instead of %.
Once you have this done, every time you want to use this on an assignment, just open this test page you made up, switch to HTML editor, and copy everything. Then go to the assignment where you want the instructions and paste it into the HTML editor. One trick if you are less familiar with HTML and need to put this into an already developed assignment...horizontal rules are an easy thing to find. Find the spot on your page where you already have the instructions. Insert a Horizontal Rule above and below the instructions. Then go to the HTML editor. Find the <hr> tag (it will be on a line all by itself). Highlight everything from the first tag to the second tag, then paste the stuff from your test page. That will automatically insert the new code in place of the old one.
Now, every time you need to make a change to the instructions, just go to One Drive or Google Drive. Update the instructions there and all of your content that uses that page will automatically reflect the changes. The above example is visually one way to do it. You could obviously use whatever style you want to mark the instructions for your students. But the steps to make it work for adjusting the height in HTML of embedded content remains the same.