It may be easier to know the common vocabulary to describe your length needs of a necklace. Use this chart to become familiar.
Choker: 14-16 inches (35-40 cm)
Princess Necklace: 18-20 inches (45-50 cm)
Matinee Necklace: 23-24 inches (55-60 cm)
Opera: 30-32 inches (75-90 cm)
Lariat: 40-45 inches ( 100-112 cm)
Sautoir: Longer than an Opera necklace