Jersey measurements on this chart represent measurements of the actual garment and not of the person wearing it. To figure out your size: take a piece that fits you well, lay it down flat, and whip out the tape measure. Compare your results with the numbers here, and choose your size accordingly.
Measure across the broadest part of the front of your chest, at armpit level, keeping the measuring tape parallel to the ground.
While standing upright, measure the distance between your waist (typically at navel or top of the hip bone level) and the top of your shoulder, to the left or right of your spine.
Standing up straight with your arms at your sides, measure from the base of your neck at the center of your spine, along the top of your shoulder and down to your wrist bone, along the outside of the arm.
Measure around your natural waistline, keeping the tape a bit loose.