VINTAGE!!!  I'm not sure if this has ever been worn as much as it's an old tshirt.  I can't find any mention of the bar "Oh My Dog & Other Little Bites" on the internet.  The scoop neck is almost 12" across.  It is 19" under the arms and 24" long.  It is super soft but I doubt very much it is cotton as it has a lot of stretch to it.  But this is SUPER UNIQUE!!

With these measurements, I would say it is a Medium rather than a Small.