Wrap a soft tape (or a paper strip) snugly around the wrist just below the wrist bone. That's your wrist size; the bracelet size adds ease on top.
How much ease
Tennis and chain bracelets: add 0.75–1 inch — it should drape, not spin freely. Bangles: measure around the widest part of the hand with fingers together; the bangle must slide over. Cuffs: wrist size plus 0.25 inch. The average women's bracelet is 7 inches; men's 8 — but measure, don't assume.