To measure neck size for a necklace, you can follow these steps:

• Use a flexible measuring tape: Get a soft, flexible measuring tape that is commonly used for sewing or tailoring. If you don't have a measuring tape, you can use a piece of string and then measure it with a ruler.
• Find the base of your neck: Locate the base of your neck, where you want the necklace to sit. This is usually just above the collarbone, in the middle.
• Measure the circumference: Gently wrap the measuring tape around your neck, positioning it just above the collarbone. Make sure it fits comfortably, neither too tight nor too loose. Ensure that the tape is parallel to the floor and doesn't sag or twist.
• Take the measurement: Read the measurement where the end of the measuring tape meets the rest of the tape going around your neck. Take note of the measurement in inches or centimeters.

Consider necklace length: Keep in mind that necklace lengths can vary depending on the design and personal preference. By following these steps, you can accurately measure your neck size for a necklace and choose a suitable length that will fit you comfortably. 

14" Collar: A collar fits snug around neck. It typically has a length of 14 inches (35cm).

16" Choker: A choker necklace is worn tightly around the neck and sits just above the collarbone. It typically has a length of 14 to 16 inches (35 to 41 cm).

18" Princess: The princess necklace is the most common length and hangs below the collarbone. It usually measures 17 to 19 inches (43 to 48 cm) in length.

20" Matinee: The matinee will fall a few inches below the collarbone. It's perfect for a plunging neckline and layering. It usually measures 20 to 24 inches (51 to 61 cm) in length.

30" Opera: An opera necklace is longer and typically hangs below the bust. It has a length of 30 to 35 inches (76 to 89 cm).

35" Rope: A rope necklace is a very long necklace that can be worn in various ways. It typically measures 35 inches (89 cm) or longer.

45" Lariat: A lariat necklace is a versatile piece that doesn't have a clasp and can be worn in different styles. It's usually longer than a rope necklace, often measuring 45 inches (114 cm) or more. 

Tip: If you do not want the chain to sit too snugly around our neck and fit like a choker, be sure to add 2 to 4 inches when you're measuring.