I had ordered and got XXL XL delivered, Claims / Exchange worked super fast and without problems. Trend-a-Porter has apologized 1000 times!
So here are the dimensions of both sizes:
Size XL:
Width (measured under the arms): ca. 51 cm
Length (center back to bottom): 65 cm
Size XXL:
Width (measured under the arms): approximately 51.5 cm
Length (center back to bottom): approximately 67 cm
BUT:
half cm difference is measured very sympathetically ... Ansich distinguish the two shirts only in length :)