in the wild you can find colonies of euphyllia that cover several football fields, but that's probably because nobody is fragging it after it grows a second branch...
I've seen a number like that in people home aquariums. Basically the person just lets it grow and grow and the new growth just keeps multiplying and in time it becomes huge. They usually don't have a lot of other things in the tank so there isn't any problem with letting it grow. This is similar to some leather corals, which can get huge over time.