I had this exact same shirt in medium recently. The fit was great on it. Slim, but not too slim. Its quite similar to a lot of the RRL westerns I have owned for a fraction of the price. Someone should jump on this.
As luck would have it, Keith wore the exact same shirt last year when I photographed them at Riot Fest here in Toronto so I zoomed in on a high res file for you. Its a vintage Sears Roebuck western shirt.