Please refer to the size chart when you purchased:
Age--------------Chest Length-------Tops Length
0-3 Months-----22cm/17.32"--------48cm/18.90"
3-6 Months-----23cm/18.11"--------49cm/19.29"
6-12 Months----24cm/18.90"--------50cm/19.69"
12-18 Months---25cm/19.69"--------51cm/20.08"
18-24 Months---26cm/20.47"--------52cm/20.47"