A pearl's size is measured according to its diameter in millimetres. Sizes range from 1 millimetre or less in the case of very tiny pearls to as much as 20 millimetres (more than 3/4") for the largest black Tahitian pearls.
The largest black pearl ever found was about 25 millimetres in diameter -- just about one inch across! Most pearls sold today are in the 7.0 - 7.5millimetre range. Akoya pearls greater than 8 millimetres are considered extremely rare. (The largest pearl an Akoya oyster can produce is around 9 or 10 millimetres.) Pearls larger than 10 millimetres are generally not Akoya pearls, but are instead black Tahitians, white South Sea pearls or large freshwater pearls.
Naturally the larger the size of the pearl, the greater its value (all other factors being equal). Larger pearls not only require larger oysters to produce them, but they require much more time to develop inside the oyster. An "average" pearl can take 2-3 years to develop. Significantly larger pearls can take twice as long. In addition, because it is difficult to maintain a perfectly spherical shape while the pearl develops, large round pearls are extremely rare and valuable.
When shopping for Pearls it is important to note that strands are rarely of uniform size. The internationally recognized standard for measuring Akoya Pearls will always have a .5mm differential within an individual strand. Per industry standards today a strand measuring 7.5mm will actually be composed of pearls measuring 7-7.5mm. This should be noted in any appraisal or description of the pearls.
The smaller pearls will be located near the clasp and the larger pearls will be in the centre of the necklace.This is considered a uniform strand, not a graduated strand.
Freshwater Pearls are more often given a full millimetre range of size. For example a strand of 8mm freshwater pearls may be measured 7-8mm in size. This is not an absolute standard, however, as many high-quality freshwater pearls are sized similar to Akoya pearls with a mere .5mm differential.
Tahitian and South Sea Pearls are typically measured individually, but to be considered a uniform strand there may still be up to a 1mm differential from the clasp to the centre of the necklace. For example a 9-10mm strand of Tahitian pearls is considered a uniform strand, but an 8-10mm strand is considered slightly graduated.