I have a long 6mm-6.5mm Mikimoto cultured pearls necklace and it's very practical. The length also makes it look more yound and modern. You can also put it around your neck and makes 2 strands. I just like to wear it really long and looks great even with just a T-shirt and jeans.
For more formal occassions, I have an 8mm cultured pearl necklace that just sits nicely on my collar bone. I think 8mm is a good size as it's not too small and not too big.