Bangles are available to purchase online here as either small or large, though really they may be better described as smallish-medium and largish-medium.
To find your fit
If you already own a bangle that fits well you can measure its inner diameter and compare it to the sizes listed here.
Otherwise one way to find your size is to cut a strip of paper 180mm (for small size) or 200mm (for large size) long, taping the ends together to form a paper bangle. Carefully ease the circle over your hand onto your wrist - it should should only just fit past your hand. If you swing your arm around does it stay on your wrist without feeling like it wants to slide off?
Alternatively you can head to your local mall and ask a jewellery store assistant to measure your size, or get in touch to have a sizing belt posted out to you or an appointment set up if you live locally.