To ensure a proper fit for a bracelet, you need to measure your wrist size. You can use a fabric measuring tape, wrap around your wrist near your wrist bone and get size. You can also use a string or a strip of paper. Again, wrap it around your wrist near your wrist bone, mark it and put it against a ruler to get your actual wrist size. See chart for reference.
Bracelet Reference Sizing Chart
The average chain length is between 16-20 inches. On an average size person the necklace will fall as follows:
15-17" Choker-Base of Neck
17-19" Princess Length-On The Collarbone
20-24" Matinee Length-Between The Collarbone and Bust
28-37" Opera Length-From Top of Bust to Slightly Below
37-60" Rope/Lariat Length-To The Waistline or Longer
For smaller or larger size necks, reduce/increase a couple of inches. This diagram is for general guidance, as sizing can vary from person to person.