Simply select a similar style of knitwear you currently wear and take two measurements:
1. Across the chest fabric, from underarm to underarm.
2. The height, from under the collar to welt.
3. The sleeve length, from underarm to cuff.
Then just compare the measurements (inches or cm) with the chart to find your size. (Note that most knitwear relax a little during wear so your own garment will have been slightly smaller when new!)
Actual Fabric Dimensions
(across the chest)
(inside, under arm to cuff)