Step 1.  Get out a 6 or 12-inch ruler.
Step 2.  Look at it.  On one side it will measure inches.  On the other side it will measure millimeters ("MM" or "METRIC").
Step 3.  Check the product description against your ruler (both diameter and width).
Step 4.  If necessary, compare the measurements to earrings you already own, to get a better 3-D feel for the size.
Step 5.  Decide whether the earrings are too small BEFORE you purchase.