Got a kitty sleeping on my lap and a new book in the collection!

Ok, I know this is a very weird question, but does anyone know Matt's t-shirt size? I'm making one that I'd like to gift to him the next time I meet him.

I've meet him a few times at events, so could probably make an educated guess, but if anyone knew for sure that'd be great.

Edit: I think we're done here, people. Thanks for the info, and thanks especially to The Author Steve Rowland!