How to measure your body: Shirt Length
Put the first end of the measuring tape at the base of the back of your neck and measure down your back to the point where you’d want the shirt to end. This point is usually the bottom of the buttocks, although you may want to go slightly longer if you plan to always tuck the shirt in, or slightly shorter if you plan to wear the shirt untucked. Round up to the nearest inch.
How to measure your body: Chest Around
Run the measuring tape around the fullest part of your chest, tight up under your armpits and over the shoulder blades. Let your arms hang down in a relaxed, natural pose. Do not puff out your chest or arch your back in any sort of pose that isn’t natural or sustainable. You have to divide by two the measure.