Mens sizes are based on the following chest measurements (from one armpit to the other):

Med - 48cm
Lge - 54cm
XL - 58cm
XXL - 66cm
XXXL - 70cm

Womens T-Shirts are a 'Skinny Fit' style. Use the following chart as an indication, that really depends on bust size:

SML - size 8
MED - size 10
LGE - size 10-12
XL - size 12
XXL - size 12-14

Kids T-Shirts are categorised into the following sizes:

2T - 32cm - 2-4 years
3T - 36cm - 5-6 years
YXS - 39cm - 7-9 years
YS - 42cm - 9-11 years