From his blog:
The HBO throne has become iconic. And well it might. It's a terrific design, and it has served the show very well. There are replicas and paperweights of it in three different sizes. Everyone knows it. I love it. I have all those replicas right here, sitting on my shelves.
And yet, and yet... it's still not right. It's not the Iron Throne I see when I'm working on THE WINDS OF WINTER. It's not the Iron Throne I want my readers to see. The way the throne is described in the books... HUGE, hulking, black and twisted, with the steep iron stairs in front, the high seat from which the king looks DOWN on everyone in the court... my throne is a hunched beast looming over the throne room, ugly and assymetric...
The HBO throne is none of those things. It's big, yes, but not nearly as big as the one described in the novels. And for good reason. We have a huge throne room set in Belfast, but not nearly huge enough to hold the Iron Throne as I painted it. For that we'd need something much bigger, more like the interior of St. Paul's Cathedral or Westminster Abbey, and no set has that much room. The Book Version of the Iron Throne would not even fit through the doors of the Paint Hall.
So what does the Real Iron Throne look like, you ask? Glad you asked. It looks kind of like this: