Extra small RF PCB antenna - GND plane
the only place in the document you link to that contains 20×40mm² dimensions is the eval board. That's just "randomly" in that size.

What's important is that the chip is offset some 1mm from the edge of a ground plane, and that you have the folded whip as specified, and that you properly match your antenna, as specified in multiple places of the data sheet. My guess is you forgot the latter.