I'm not sure what size should I order. I'm...
I'm not sure what size should I order. I'm 55.5 cm. I should buy S size, instead of M?
I am pretty sure you are a Medium.
Check the size chart. Probably Small or Medium.
Smith Helmet Size Chart
Measure the cirumference around your head to determine the correct size.
Holt and Maze
XS S M L XL
Head Circumference 20.5-21.25in
Other Adult Helmets
S M L
Head Circumference 20.1-21.65in
One Size Youth S Youth M
Head Circumference 19-21in