How long typically are paper reviews? Is there such a thing as “too long”? When you review papers submitted for publication, is there an “optimal” length for reviews?
In my experience as an author and referee, I have seen a large range of review lengths (for reference, a paper in my field is typically between 3 and 8 printed pages):


Zero length: for some of the papers I have authored, the reviewer just clicked the “accept as is” or “reject” checkmark on the review sheet, without adding any comment (at least, not any comment visible to me). It's not so common, but it has happened.
Short length: a lot of time, I received reviews who consisted of a single paragraph.
Medium: one full page, maybe two pages.


Although I have never received any such long reviews, I have myself written on a few occasions reviews that exceed two pages, including once or twice a four or five-page review.  These were cases where the paper was good, but could be (in my view) much improved and some of the aspects/consequences had escaped the authors' consideration.

I am wondering how useful reviews of various length are to editors. I often consider that “unmotivated” reviews are useless, as they do not give any real insight about the paper to the editor. For example, if the editor gets two conflicting non motivated reviews, how is she to decide?

On the other hand, although I write some from time to time, I have never received long reviews, so… is this something frowned upon?
Speaking from the point of view of an editor: One of the best reviews I ever got was longer than the paper. The author, a young researcher, had proved three theorems, one of which I recognized as a known result. So I asked the original discoverer of that known result to referee the paper. In my cover letter, I mentioned that I recognized one of the theorems as his, and asked whether the other two theorems had enough novelty for a publication. It turned out that the other two theorems weren't new either. The referee could easily have just given citations for those two theorems and recommended rejection. Instead, he gave me (or, really, gave the author) a long, clear explanation of the state of the art in that subject, and he suggested some open problems that the author could try working on.