Along with the images of the apparel, each item will have a sizing chart. This image may help understand the measurements on the sizing chart:
Body Width (Körperbreite): Abstand in Zoll von der Naht, an der der Ärmel auf jeder Seite mit dem Mittelteil verbunden ist. (Measure from armpit to armpit)
Tip: Measure a well-fitting shirt or sweatshirt you currently own. Lay it flat and measure from side to side just underneath where the sleeve connects to the body of the garment and compare with the WIDTH on the size chart.
*Due to the nature of t-shirts & apparel production, each item does have a 1/2 inch tolerance which means it may be slightly bigger or smaller in either direction*