Before we go any farther, you should know that the most popular, classic and versatile pearl necklaces are in the 7.0-8.0mm size range. The 7.0-8.0mm size range is widely acknowledged to be the "go-anywhere" size, perfect for nights out, office wear or even a quick trip to the grocery store in a T-shirt and jeans.
This size range is also age-appropriate for ladies from 16 years old up through women in their 50's, so these are necklaces that will be worn and enjoyed for many, many years.
Each Pearl types their sizes are quite different:
It is important to understand that the size of cultured pearls is depended upon factors like the timeframe of staying underwater, climate, plankton’s nutritional quality, etc. The bigger the size of the pearl, the more valuable it is going to be in the market. On the other hand, the shape is dependent upon nothing but the randomness of our natural world. Some of the shapes and sizes that cultured pearls come in include a spherical, semi-spherical, button, drop pearls, oval pearls, etc.