How big can supertankers get?

I believe they can't be too big because there comes a point when they steel structure cannot support itself in the water properly and the waves literally break it in two down the middle. The largest supertanker, and ship in general was the Jahre Viking, which was built in 1979 and scrapped in 2010. She was a bit over 1,500 ft long, or about as tall as the Sears Tower in Chicago if you stood up her next to it. The largest supertanker still in service is around 1,200 ft long. But I don't think they'll get much bigger than 1,200 or 1,300 ft. The era of big supertankers was the seventies, and some of the largest ships in the world were built back then. Almost all of them have been scrapped. But to answer your question, although it's the cheapest way to transport oil, 1,200 feet long or even 1000 ft is plenty long enough.