All clothes on this website are listed with European sizes and age estimates. The European size correlates to you child's height in cm (e.g. they will fit in a size 80, when they are around 80 cm tall). The clothes fit up to the size (e.g. a size 80 will fit up to 80 cm. After that they will fit into 86).
Beanies are listed in cm and age estimates. The cm's indicate head circumference.
I personally prefer the indication of sizes in cm, because it gives a better idea of what size a child actually is at the time. Both of these measurements are taken at the doctor during regular check ups as well and the parents might know them because of that. If you are looking for a gift the age estimates are more helpful to get an idea of what should fit best.
Please see an overview showing both measurements below.