Dezzo,
I can't help with the question you've posted, but just in case you aren't familiar with the Diffenbachias, they are usually listed on poisonous plant lists, and can affect children and pets as well. I just wanted you to know in case you didn't.

Rodden-Blessed

Dieffenbachia seguine Maybe? A 6" stem diameter is probably unlikely though you can get the height and leaf size you're talking about.

These plants, as well as all aroids do contain toxins to some degree. So do many other common house plants, Most concerns are overrated but still keep an eye out for house pets and kids munching on your plants.

Almost four years after the question ... I hope it's still useful.
I think I know this plant, which I could not buy; was among those that the Flora Novaes nursery - Campinas, São Paulo, Brazil - did not sell. There they called it "Dieffenbachia cubana".
I discovered later that this is a variety, called "Big Ben". Leaves green, without spots or variegation, and the whole plant very large. I think it's a hybrid.
Good luck!