The wool cover is available in the sizes: Size 1 (ca. 4-7 kg), Size 2 (ca. 7-12 kg) and Size 3 (ca. 11-16 kg), and as a one-size nappy (ca. 7-16 kg). The one-size cover corresponds to Size 3 with reduction buttons on the front and can be, at least at the beginning, too large for very slim children.
Note that the weight information of the sizes can only be an estimate, as the fit depends on the stature of the respective child.
The type of outer fabric also makes a difference on how the nappy fit. Wool covers made from stretchable fabric tend to adapt better to the child’s stature and are suitable for stronger children. According to experience, they also fit longer than covers made from non-stretchy fabrics (e.g., pure cotton) and this is especially important for newborns. Non-stretchy covers on the other hand are more likely to retain their shape and are recommended for slim children. If there are only a few but very absorbent inserts, wool covers made of non-stretchable fabrics are more suitable, as they give the nappy stability.