Bangle size is measured by the size of your hand.
To measure your bangle size, tuck your thumb into the palm of your hand and using a thin rope / thread, measure your hand all the way around the set of knuckles closest to your wrist as seen in the image.
Keep the thread / rope in a ruler and find the length. Compare it with the below chart and find the size.
Here is another method to find the bangle size if you already have a bangle with you which fits properly. Measure the inner diameter of the bangle using a ruler. It gives the measurement in millimeter. Compare it with the below chart.