My last post was on the importance of speed, i.e., timely feedback in online courses. In this post, I'd like to make some comments about the amount of feedback. Of course, the amount and details of your feedback depend on your course, whether it is a graduate-level seminar of nuclear mechanics or a musical composition course or an introductory survey to western history.
Your feedback should be appropriate for your course goals/objectives and appropriate to your audience. Let me restrict myself to the survey history courses that I teach. If my primary goal in the courses is to teach critical analysis of primary sources, then feedback that targets a student's analysis is most important, and red-pen markups of grammatical errors on a paper is of secondary importance. Yet what students see when they look at their feedback is the grammar corrections. They are not going to get the important feedback that you are trying to give them about developing analytical thesis points and then supporting that analysis with evidence.
You do not want to swamp students with feedback, because then a student will be less likely to discern the key parts of the feedback. A student is not quite sure what you are really asking them to focus on. So, for example, on a typical, five-paragraph, or one-page paper, feedback might be 25 words max. Short, succinct, and to the point.