There is a certain size. They have a measuring chart on there website that tells you the best way to measure yourself. Its sevenslings.com The package says a conventional size Large. Womens T-shirt size L-XL but the best way to measure is from Left shoulder to right hip going across your stomach for inches. I do have an order form somewhere that I can swap out sizes for free one time, I just have to find it. Let me know if you need anymore help.