@kewo1 ...
I'm not aware of any limit to the length of a content page within Canvas. (Though, I'd be happy to be wrong.) I also don't see anything about limits in this Guide:
How do I add and modify text in the Rich Content E... - Instructure Community (canvaslms.com)
(By the way, the "built-in editor" you refer to is called the Rich Content Editor ... or RCE.)
Has your teacher considered breaking down the page into separate content pages? I ask because you should also be considering mobile users...especially those using smart phones and tablets. As you may know, what might look really great on a desktop/laptop computer will take lots and lots of scrolling on a mobile device...much more scrolling than you would do on a desktop/laptop computer. So, if your teacher can somehow break down the current page into smaller chunks, that might be a more welcoming experience for all involved.
Hope this helps a bit!