If you wear a 12 mm 18" strand, it says "Look at me." Depending on what you wear it with, it can look like high end costume statement necklace, or a Ladies who Lunch staple. The clothing determines the meaning.
A 20" double 7 mm is "Jackie O." Personally, I think it is the most versatile piece of fine jewelry. It won't provoke comment no matter what you wear it with. But it is obviously expensive and conservative. It defaults to 25-45.
A vintage graduated strand is "grandma's favorite granddaughter." If you want to look sweet, innocent and family-values centric - it is fabulous. if you wear it alone, it will have a hint of childishness/old lady. This one is actually more aging than the big pearls.