Current large whales need enormous amounts of insulation (a big part of why they were hunted a hundred years ago) to maintain their body heat. While there is n

Current large whales need enormous amounts of insulation (a big part of why they were hunted a hundred years ago) to maintain their body heat. While there is no doubt a point where heat becomes an issue for sea creatures, it's a ways away. Given the degree of overfishing we've seen in much of the world and depletion of the fish stocks, I think a more realistic upper bound would be the ability to consume enough mass. Although on an evolutionary time scale maybe fishing policy isn't as much a problem.