We recommend choosing the smaller size if you are struggling to choose between two of them. Our sleeping bags run big. This size guide based on height might be helpful:

Size 0 months: between 40 and 56 cm.
Size 0 - 6 months: 
between 56 and 68 cm.
Size 6 - 18 months: between 68 and 82 cm.
Size 18 - 36 months: between 82 and 95 cm.