It's a really small cup - like between 8 and 10 oz.
I chugged mine on the way to the food court to fill up my 16 oz Rapid Refill mugs with more coffee !
Answering my own question... I brought 12 oz to go cups from home just in case and they're exactly the same size as the ones in the room. So at POFQ anyway, the cups are 12 ounces (and work perfectly as oatmeal cups!)