Leslieray, that is indeed one very impressive looking Tephrocactus! Love it! I guess given the right conditions, it can indeed grow to such awesome size even in a container.
I have one, just do not know what type of Tephrocactus it is exactly.
To show how my plant looked like when I acquired it last April 2017 and still maintaining it in the 4 inch container it came in:
Currently, after overwintering indoors, it is now outdoors once more this late Spring: