Sizes:
7-8 years
9-11 years
12-14 years
For larger sizes see adult small which is similar in chest size to the largest youth t shirts but a few inches longer.
Measured laid flat (and there is quite a lot of variation across t shirts)
7-8 years armpit to armpit 42cm, back of neck to bottom 52cm 9-11 years armpit to armpit 46cm, back of neck to bottom 56cm 12-14 years armpit to armpit 50cm, back of neck to bottom 60cm