How long typically are paper reviews? Is there such a thing as “too long”?
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.