Is it Sinnoh,Unova,Kanto,Johto, or Hoenn?
In size I would have to agree with Adam and say that Sinnoh is the biggest in size mainly because the Sinnoh region is based on the island Hokkaido which is the largest ins size compared to the other locations that the other regions are based of as seen in this picture:
However the region that is largest when calculated by population is the Unova region that counting 804 inhabitants
Sinnoh is the biggest region of all (if I knew how to place pics I would).
hope I helped!