Yep, that and other pages tell me he is medium. A large, as in big, medium, but nowhere near the as much size difference as between humans and cats.
I wouldn't even say Powerful Build. He's never been shown using a weapon that would be ungainly in a medium creatures hands of his apparent build.
I will see if I can get Thunt to make a confirmation in writing. He's said it in the live-stream before, I know it. I had commented about the size of Dies and Fox and how normally they don't look so short because they're all goblins and there's no frame of reference, so when they are next to the medium orc it really brought home how short they are. The response I got is that Biscuit is large, so that's part of why there was so much difference.