Most Only Child pieces are designed to be roomy and slightly oversized. If you have questions about finding the best size for you, please feel free to shoot me an email at email@example.com. I recommend going by the measurements listed in the product description. If you're not sure how to take your measurements, check out this handy guide.
In general, size S/M is meant for sizes 0-6 and M/L is meant for sizes 8-12. SIzes S, M, and L are true to size, you can get some more information from this sizing guide: