Standard lengths
Common lengths are 16, 18, 20, 22 and 24 inches, with 18 inches the usual retail default for pendants. Choker and layered styles use shorter lengths and often an extender.
Measure a necklace end to end including the clasp, and state whether the clasp is included in that figure.
How length affects fit and cost
Length changes fit and metal weight at the same time. A 2-inch increase adds chain weight, which shows in the price per piece.
An extender can cover two lengths with one SKU, at the cost of extra findings and a less clean look.
Specifying length in an order
Keep one length per SKU and record it in the product specification. Mixing lengths inside a SKU creates picking errors at packing and returns at retail.
State a tolerance so a measured shipment is not rejected for a millimetre, and confirm the clasp type at the same time.