How do I know which size Nature Babycare Organic Diaper I should use for my child? It is not uncommon for some children to experience a less than ideal nappy fit although they may fall within the recommended weight range of a particular diaper size. Babies come in all shapes and sizes, and although their weight may be within a certain range, their body shape can vary significantly. The only practical solution is to try a larger or smaller size of nappy to achieve an optimum fit.