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.
This list flows from thickest needle to the thinnest, the higher the number the thinner the needle.
18 Gauge = Pink
19 Guage = Cream
20 Guage = Yellow
21 Guage = Green
22 Guage = Black
23 Guage...