Whether you're purchasing a bangle, bracelet or cuff, you want it to fit comfortably! Bangles need to be large enough to fit over your wrist without a clasp, bracelets need to be securely closed and cuffs should lay comfortably on your wrists without falling.
Close your fingers together and bring your thumb to your little finger. Pretend that you were putting on a bangle / bracelet / cuff.
Using a strip of paper or tape measurements, wrap it around your hand at the widest point.. If you are using a plain strip of paper, mark your size with a pen on that paper. Then measure that paper strip with a ruler. That would be the circumference of your hand.
Purchase a bangle / bracelet / cuff with diameter that is the next size up from your actual wrist measurement.