He looks so average (height) to me on GoT...
I see him on screen and he looks a good solid, honest 5'10 to me. But then I see him in candids or award shows and it's obvious he's part of that cadre of actors who inflate their height. He's 5'8 at most, and a stocky 5'8 at that. It must be either heels on the boots or he wears lifts on the show to make him taller. (All those layers make him look bigger, too.)
Still a good-looking kid, but he's wee. In the new Men's Journal fashion set, you can see he's built pretty broadly at the top, but is a short guy overall.