Whether ordering a bangle for yourself or someone else, here's a brief guide to measuring bangle sizes:
1) Squeeze the hand you plan to wear them on tightly and narrowly together, as if you're sliding on a bangle. See picture.
2) Use measuring tape (or string and a ruler) to measure around the WIDEST part of your knuckles since that's the widest point that the bangle will need to fit over. Wrist measurements don't pertain to bangle size at all. On average, the most common size is 8 inches, which we call a "medium". Ultimately, remember when ordering that you do want bangles as tight as possible so that they don't come close to falling off when you're wearing them.