Size dramatically affects the the price of pearls, especially above 7mm. People usually buy the largest pearls they can afford, but then again, it’s all about personal preference. Large pearls are rarer and usually require a longer time to mature. the largest pearls are from the South Seas and Tahitian waters.
Pearls can range in size from 1mm to some of the largest at 20mm. 15mm to 20mm are very rare pearl sizes and maybe never seen in a fully matched necklace. There are several famous pearls that are larger, but that is another story. the size or weight of the pearl is the only grading factor that can be measured and can be related in exact figures.