The appropriate length for gold necklace stacking sets varies according to personal choice and the desired aesthetic. For a balanced and visually acceptable layered look, it's best to blend different lengths, such as 16 inches, 18 inches, and 20 inches. This arrangement allows the necklaces to complement one another without getting overly crowded. There are no hard and fast guidelines, and the optimal length ultimately relies on your style and the design of the necklaces you're stacking.