Bangles are measured by their diameter, which is a straight line passing through the centre of the bangle, finishing at the inside edges of the bangle.
To find the size of bangle which will fit, you can measure an existing bangle or measure yourself using a flexible tape measure. Put your hand flat and then bring your thumb (in your palm) and your little finger together. Then wrap the tape measure over the top knuckles of your fingers and pull tight (this will be just off your top thumb knuckle). The top knuckles, being the knuckles nearest your wrist.
Once the bangle is over the knuckles of your hand, then of course it will be relatively loose around your wrist.
Do remember that your hands could be different sizes, so ensure you measure the hand on which you intend to wear the bangle and to ensure that the tape measure is straight across your knuckles, because the bangle is not flexible.
To give you an idea of the size of bangle to purchase. If you measure your knuckles, are pulling the tape measure very tight and the measurement shows 19cm, then the S - M will be a good sized bangle for you. If, when pulling the tape measure tight, the measurement shows 19.5cm, then S - M will be difficult to slide over your knuckles and you need to be looking at a M - L sized bangle.