Please measure around the largest part of your chest to measure its circumference. 
MEN
Measurement (CM) XS S M L XL XXL
Ideal fit 84 ± 3 90 ± 3 96 ± 3 102 ± 3 108 ± 3 114± 3

  

WOMEN
Measurement (CM) XS S M L XL XXL
Ideal fit 84 ± 3 90 ± 3 96 ± 3 102 ± 3 108 ± 3 114± 3
 
All Possenia products are designed to be race fit 'ideally' thus, they are snug fitting and streamlined along the body. If you prefer a looser fit, take one size up. If you prefer a super race-fit, look at the higher ends of the ideal fit i.e if you're male, measure 98cm around the chest and want a serious race-fit, take an M.