Cotton shirt

Chest size: to measure this, place your measuring tape where the clothing chest is widest. 

Length from the shoulder seam: you measure this starting from the seam between neck and shoulder and the bottom of the head

Back shoulder length: this measurement is calculated by looking at the distance between the endings of the two shoulder seams.

Arm length - including wrist cuff: this measurements is the length of the whole arm, including any wrist cuffs. This measurement is made starting from from the shoulder seam.