You may think that eyebrow rings only come in one size, but think again. The standard gauge for an eyebrow piercing is a 16, but depending on what type of jewelry you were initially pierced with and how it healed, about one size deviation in either direction is possible.
The other thing to consider is length. For all curved barbells, length is measured as the distance directly across from tip to tip, not as the full length measured along the curve. There are standard sizes here too (most eyebrow jewelry comes in a 5/16 length, which is about 8mm and generally adequate), but this will again vary slightly, and should be taken into account.
For those who wear circulars in their eyebrow piercing, like ball captives and segment rings, the most important measurement to know will be diameter. Captive rings will come in anywhere from 6 millimeters in diameter to 12, and the sizing makes a definite difference in comfort, aesthetics, and fit.
The easiest way to figure out what size you need is to go to your piercer and have them tell you, but its also possible to measure an item you already own. To do this, choose an item that fits you perfectly and that you like the look of, make sure both tips are fastened securely, and measure with thin thread or against a sizing chart.
Once you know your exact size, the sky is the limit on jewelry choices.
For more sizing information, check out our Jewelry Sizing category.