^^^ No spoilers but here's some book knowledge relevant to the recent line of inquiry. According to the books, a dragon will not stop growing as long as you feed it and give it room to grow. The old Targ's (Dany's ancestors) stiffled the growth of their dragons once they had reached their desired size by housing them indoors; when they weren't riding them.
As seen on the ship a few episodes ago, Dany tends to let hers roam pretty freely, so they should be quite large if they make it to adulthood.