Usually the larger the pearl the more it costs. Pricing often depends on availability & demand. Weight, Length Size, the size of round saltwater cultured pearls is expressed in terms of their diameter measured in millimeters. One millimeter is about 1/25 of an inch. Since pearl size varies within a strand, a range of 1/2 millimeter is usually indicated, e.g. 7- 7 1/2 mm.
Occasionally, a few of the pearls might fall slightly above or below the size indicated. Usually, the larger the size, the higher the price.

The size of non-round pearls can be expressed in terms of their greatest width and length and in some cases depth. The measurements are generally rounded to the nearest half or whole millimeter.

When determining the effect of size on price, keep in mind the following.
• Price jumps between pearl sizes are often uneven. As the sizes reach the 8 or 9 mm mark, pearl prices tend to jump more.
• Price/size relationships can vary from one dealer to another.
• The effect of size on price varies from one harvest to another.

If too many pearls of one size are harvested, their price will go down. 4 Demand can have an important impact on the way size affects price. If there's a high demand for a specific size, its price tends to increase. This explains why occasionally smaller pearls sell for more than bigger ones of the same quality.