it is more correct to compare with the height:
Unit：CM
S：length=65 width=37, fits for: 90-110cm height / 2-4 Years Old

M：length=70 width=38, fits for: 110-130cm height / 4-6 Years Old

L：length=75 width=39, fits for: 130-150cm height / 7-10 Years Old
Note:
1.There is 2-3% difference according to manual measurement.(1 inch = 2.54 cm)
2.All measurement in cm, please note 1cm=0.3937ch, 1 inch=2.540cm
3.Please check the figures carefully and order the right size, don't completely depend on the " age advice". It's just for reference.