Place the end of a measuring tape beside the collar at the top of the tee (high point shoulder). Pull the tape to the bottom of the shirt.
Measure yourself around the fullest part of your chest. Keep the measuring tape horizontal.
C Sleeve length
Place the end of a measuring tape at the center back of the collar, then pull the tape along the top seam of the sleeve. When you get to the shoulder hold the tape in place at the shoulder and continue to pull down the sleeve until you reach the hem of the sleeve.