GM lists the fuel tanks to be 17.5-18 gals. Not sure if that is without the fuel pump in it. Or just the tank. Regardless, that is the size.

For me, my light comes on when I have used around 13 gallons. But I can easily keep driving and put 15 gallons in or so. The tanks have a reserve and the light comes on early so you don't run out. Typically I get 320-360 miles on a tank. I just filled up last night with gas and put in 14.3 gallons, and had 347 miles on the odometer.