Posted: 24/08/2013 at 19:44
I grow Russian Giants which, in our sandy loam get to about 7ft and the size of a breakfast plate if we're lucky. However, I know people who grow them much bigger than that. I think they're probably the most usual variety grown for size in the UK.
I think growing conditions have a great deal to do with the eventual size as well as variety. You've obviously been selecting seeds from the best plants, and as this article states, it's as much to do with selection for strong stems as height and size of flower
Goodness knows what type the original one you grew was, but as the years go on you're developing a refined selection