This fascinating bit from Dr. Nick Pyenson in the New York Times explores why whales, which have an earliest-known ancestor not much bigger than my dog (although probably less neurotic) became the giants they did. We're generally conditioned to think of giants as things of the past (e.g., dinosaurs), but whales are the biggest they've ever been. Dr. Pyenson believes whales exploded in size (cue joke about that exploding sperm whale in Taiwan) when the first Ice Ages changed the way their favorite prey was distributed, concentrating it in warming waters and essentially giving them such a buffet that limited food was never a factor. (See the study he and co-authors did on that here.)
So could they get bigger? He thinks not, citing studies indicating that the biggest blue whales are about as big as they can get and function physiologically. I think of airplanes as an analogue: you can design a 1,000-ton airplane, but the limits of materials, potential maximum engine efficiency, and so on mean you couldn't build a workable example.
Pyenson also notes whalers cropped the biggest giants for a long time, and modern species might be a BIT smaller than they might have been. While some big whales are doing ok, some are not (big animals are more likely to get hit by ships, caught in fishing gear, etc.) and smaller cetaceans are still in serious trouble: one example, the little porpoise called the vaquita, is facing extremely steep odds of survival after a captive breeding venture failed and the population has been estimated as low as 12. We can still save most species, if we choose to.