Pick out an old shirt that fits you well and lay the collar flat. Measure from the center of the collar button to the far end of the button hole. The neck size is the number of measured inches. Keep the measuring tape up under your arms and measure around the biggest part of the chest, letting the tape go around the shoulder blades. Measure from the center of the back of the neck to the wrist, with your arm slightly bent.
Arm size is the number of measured inches.
Keep one finger between the tape and your body. Stand with heels together and measure around the fullest part.