You don't really see his size in this episode compared to other people. Conan Stevens is 7'1, I believe, whilst Gregor in the books is around 7'6. A noticeable difference, to be sure, but he's still the tallest person in the cast by a long shot (helped by the fact that a lot of the other tall characters, like Robert and Sandor, are shorter than in the book). They did interview taller actors, but they were pretty gangly and couldn't bulk up to the Mountain's size in time, whilst Stevens, as a bodybuilder and stuntman, was already the required shape.