There is a size guide tab on all Active Hands product descriptions – you can use this to help you to decide which size to buy. We can exchange items that are the incorrect size as long as they are still in saleable condition with all packaging and labels. If your measurements fall between two sizes then please purchase the one that is closest to your size. If this does not fit then we can exchange for the next size as long as it is in saleable condition. If we do not sell a product that fits your measurements (for example if your child is too small to fit our mini aid) we offer a bespoke service. Please note that there is extra cost and time delay associated with this.