Typically, most White or Golden South Sea pearl necklaces - both round and baroque shapes - are laid out in a “Graduated” design.
This means that the pearls will begin near the clasp at their smallest size, and gradually increase throughout the layout, culminating in nice large center section.
Traditionally, these graduation rates span 2.0-3.0mm on average, so you'll often encounter necklace descriptions with measurements reading 10.0-12.5mm, or 9.0-12.0mm and so on.