Like zak said, you can get whatever size you want, depending on how much torque or top end you want.
The stock is 16t, so I went with an 18t, which made it a lot faster AND still has a ton of torque. The Volcanos were designed with ALL torque right out of the box, plenty to spare. You will be thrilled with an 18t...

You will have to completely start from scratch when tuning. My settings wee WAAAAY off when I changed over, but that's normal and tuning is simple.