Purchased a large for my 11.5 year old son who is thin and 5'1" tall. Appears to be nearly a perfect fit being about 1 inch long in the arms with the length about mid-thigh area. Should be able to get at least one more year out of this which is what we were hoping for. We made sure to purchase a Columbia jacket with the Omni Heat Thermal lining because my son's previous jacket was the same brand with the same lining and it served my son very well the times he wore it in the NC mountains when tubing. The jacket is thing and lite; but, the Omni Heat Thermal lining works very well in keeping one warm by keeping in ones' body-heat. The funny thing is my son wears it to school here at home (currently 53 degrees) in SC and keeps it on all day because he likes the large internal pocket whereby he can insert a few notes books and whatever else he desires.