It is 1,579 km². If it were a country, it would be the world's 181st largest country in area. Okay, so it's larger only than very small countries. But still.
I really think of Greater London as being everything within the M25, which is a slightly larger area than that actually covered by the boroughs of Greater London. But while I can find the M25's length easily enough, I give up, for now, on finding the exact area it encompasses.