A discus can be considered full size at around 16-18 months.
Getting them over 8 inches requires a lot of effort and often is not possible unless you yourself are breeding the fish that you're growing out.
It requires dilligent care and absolutely pristine water from day 1 along with an experienced feeding regimen - nitrates literally must be below 10 ppm for nearly the first year of the discus life.
That's not to say that it isn't possible with discus you've obtained through reputable breeders, but it's unlikely.
As in all fish, a good way to tell if a discus has been stunted is to compare the size of it's eyes to the size of it's body...