Pearls, are always measured in millimeters, by their smallest width, shown in the picture above. Typically sizes are generalized, meaning that an 11mm pearl, can be anywhere from 11.00-11.99mm.
Pearl Size Ranges:
Different pearl types occur in different size ranges. For example, a 9mm pearl is considered to be exceptionally large by Akoya standards, but by South Sea standards it considered to be small.
More often than not, a strand of pearls will gradually vary in size. Typically, this means that the center pearl is largest.
Smaller pearl types (Akoya and Freshwater) are usually lightly graduated about .05-1mm. (6-6.5mm). Larger types such as South Sea are more typically graduated from 2-3mm. (12-14mm)