I have to also give a nod to Slankards since it is not only a very large tomato, but a very tasy one as well. Stingy on production here in the South but well worth a spot in the garden.
Have you seen Golden Dome? It is another one that without any effort on my part made some really big tomatoes.
http://doublehelixfarms.com/golden-dome
Make sure you click the picture. There are 4 more pics on the page and one of them is a photo of a really big one.