Did you write it in the assignment comment box on the right, or as an additional comment, which appear under each question (if it was a quiz)?
Generally you can write up to 65,000 characters in submission comments, which I think is what would apply here, so that should not have been an issue at 5 paragraphs: Canvas Character Limits - Instructure Community - 529365
There was an issue with saved comments/rubrics a year ago, truncating comments to 99 characters, but that seems to have been fixed: Re-using rubric comments longer than 100 character... - Instructure Community - 610424
