Many are unsure what size pearls to buy.  This pearl size comparison chart can help to demystify the sizing of pearls.  Small pearls would be about a quarter of an inch in diameter and would be in the range from 6.0mm to 7.5mm.  Medium sized pearls would be about a third of an inch in diameter, which  would include pearls in the 7.5mm to 9mm range.  Large pearls would be over 9mm in size.  Whether a pearl is considered large or small would also depend on the type of pearl.  A 9.0mm (just over a third of an inch) would be considered very large amongst white Akoya pearls, which are sometimes known as cultured pearls.  They are the classic high quality white pearl necklace and few pearls grow to that size.  However the same size  in a white South Sea pearl would be considered relatively small because the South Sea pearls grow to huge sizes (up to 20mm or about 3/4 inch in size).  The reason for this is that the gonad, where the pearl grows, is about four times as big in the South Sea oyster as the gonad of the little Akoya pearl oyster that lives around Japan.  If considering what size pearls to buy, remember that 6.0-6.5mm pearls are a bit small for a mature woman, unless she likes a dainty style.  They are better suited to young, slender girls.  Some prefer to buy a size larger, 6.5-7.0mm, even for a younger girl, so she can enjoy them for many years.  Sizes between 7mm and 8.5mm are the most popular in the traditional white pearl necklaces. An average size for larger pearls like the South Sea and Tahitian pearls might be about 12-13mm, which is about half an inch in diameter.