18 gauge. that's the same gauge as a standard earring. There is also a 20 gauge which is a little smaller than an 18 gauge and rarely used for body piercing.
Bigger than a 14 but smaller than a 10. Its a fairly large needle. To get a good visual go into a store that sells gauges like Hot Topic or any piercing studio should have some.
The smallest gauge needle that I'm aware of is 22g. But no one will pierce you with anything less than 20g, and that's usually in the nose. But, the standard size piercing needle is around 16-12g,...