I believe they are 18 gallon, but my gauge makes me fill up way before it's empty. I regularly put in over 10 gallons. Mine shuts off correctly, but I try not to fill it up to that point because I get a strong fuel smell in my garage because of the venting. I don't seem to get it when I fill it to the point I can see the fuel at the bottom of the neck when filling.
If you measure your tank and use 1 Cubic Inch = 0.00432900433 Gallons, it comes out to about 18 gallons by my pea brain.