"This review is more than I can digest!" said Goldilocks, as she sampled Papa Bear's detailed critique. Next she moved to Baby Bear's terse response. "And this review has no substance!" she complained. "'Needs work' doesn't feed me at all!". She walked around to Mama Bear's response. "This one is just right," she beamed happily. "Now I know just where fo focus my efforts!" How much is too much? How much is not enough? If you are the reviewer, you don't want to spend an hour and a half writing a detailed review that the author skims over but doesn't know where to begin with. And the author is likely to be discouraged by a laundry list of small details as long as his or her arm. Of course, too short a review isn't helpful either. A review needs to be specific, and if it addresses minor punctuation defects when the overall structure is the major flaw, you may as well not have bothered. The piece you are reviewing may need improvement in many areas (in your opinion! Never forget that!). But to keep a review to a digestible size, you should stick to no more than three top five points. Why three to five? Because that is a quantity of information easily grasped and held in someone's attention. More than that is like trying to carry too many eggs across th eroom in your hands; you are not ony likely to drop the excess ones, you are likely to lose the lot of them trying to hold onto that most precarious one. So because you need to focus on a small number of points, be smart about selecting them. Don't just latch onto the spelling mistakes because they are easiest to convey. Try to select the three to five biggest problems with the piece. Don't try to find every example if it's a recurring item, just point out a couple exaamples and mention that there are other instances. This way, you should be able to generate reviews that are reasonably concise, but more importantly, cut to the heart of the matter with useful recommendations.
By the way, the number of points to address does not depend greatly on the quantity of writing submitted for review. Whether it is short or long, limit the suggesteions to three to five points, and no more than 3-5 examples for each point. That doesn't mean that if you only find one point you believe can be improved, that you must find a couple more. It just means don't overwhelm the writter with more than he or she will be able to work with. You aren't editing. You are helping the writer with ideas to improve the piece himself or herself. It's up to the writer to make the changes, or not, and to apply it to future work (or not). The writing you are reviewing is a sample. It may or not be complete in itself. If the writer gives you a larger sample, you are in a better position to analyze flow, character development, plot, exposition, and the like. But it also means you will have more reading to do before you can approach those issues; the writer's risk is that fewer reviewers will go to that effort.
The moral of the story is, "Don't load more info onto the writers at once than they can bear." (Sorry, I had to do it )