The larger the size of the Pearl, generally the more valuable it is. Now this is a broad statement since there are many other factors that influence the value of a pearl gem such as the Pearl Shape, Luster, Color and state of the Body Surface. Pearl jewelry is measured by the diameter of the pearl. In the case of a non-symmetrical or an off-round pearl, it is measured from the furthest points on each side of the pearl.
High quality freshwater pearls have a tolerance of up to 1mm in size. These size differences should not be alarming to the consumer as it is extremely rare to fine pearl necklaces with exactly the same size pearls strung from end to end. The size difference is generally not noticeable when worn. If you do fine one, then it may likely be an imitation. Remember Pearls are like snowflakes. They are all different.
Tahitian Pearls and South Sea Pearls also have a tolerance of 1mm which is considered a uniform strand. However it is common that these strands be slightly graduated i.e. 9 to 11mm. This will be a graceful graduation from each end to the center (largest).