If I recall correctly, I looked it up recently, it's 11.9. On another thread we decided (my biased interpretation) that Prius drivers are obsessed with getting the most miles out of a tank... we wait until the very last minute to add fuel even though the recommendation for this (and most) cars is to fill up at a quarter of a tank!
it is a 11.9 gal tank but the bladder inside the tank makes the fuel gauge a bit unreliable. It usually blinks early but the problem is that there's no set distance that you can travel with a blinking low fuel light. Some have gone 100 miles, others only 10. My suggestion is that once it's down to one bar, just start looking for or plan to stop at a station as soon as it starts blinking
You're right, it's a silly question. It's been asked only about a gazillion times. The tank holds what it holds. You're getting good mileage. There's no way to increase it's capacity. It's not broken. Fill up when it gets down to 2 bars and you'll be fine.