The grey part of the brain is folded to fit inside the skull and, if flattened, it would cover the surface of an office desk.

The cerebral cortex (grey matter) is indeed highly folded with gyri and ridges to maximize surface area within the limited skull space. When unfolded, the human cerebral cortex would span approximately 2,000-2,500 square centimeters, roughly the size of a large office desk or small table. This folding dramatically increases the brain's processing capacity.