Wearing a properly sized shirt displays an impressive sense of fashion and professionalism. Long sleeve shirts are sized based on three measurements: Chest size, sleeve length and neck size. For a truly professional look, you should have a shirt tailored to fit your exact measurements. However, if you do not have that luxury, then you can opt for the closest match.
Measure your chest around the widest section, close to your armpits. You should take a breath to measure take into account flexing when your breath.
Bend your elbow and hold your wrist right about waist level. Measure the distance from the center, back of your neck, over your shoulder, outside the elbow bend and to your wrist.
Measure your neck around the center. You may wish to add a half and inch to the measurement for comfort.
Refer to your prospective shirt's size and closely match those measurement. Most shirts will display the neck size on top with the sleeve length below. Some shirts may also display chest size.